At last after a couple of problems this is now fixed and it turned out to be quite a simple fix in the end, thought I'd write a brief summary of what I did to help anyone else out who may have this problem in the future. I should have taken more pictures but I just wanted to get on and get it done. There are a couple above I took earlier on in the project.
Firstly I removed the entire suspension leg and driveshaft as one unit to give me space to work in, this is a fairly simple job, 4 bolts under the bonnet for the top of the strut and 2 lower arms to unbolt from the sub-frame. You obviously also have to remove the brake caliper but there is a snag, the brake line is routed through the strut and is trapped, so I had to split the strut at the top by unbolting the two upper arms and letting it hang forward, I then just tied the caliper up on the right hand side out of the way. The driveshaft bolts to the flange on the differential with 6 12 point spline or triple square bolts, think they were M10.
Once all the suspension was out of the way you can see the front cover to the diff quite easily, its held in place with 10 triple square bolts M10 again, At this stage you can either undo these and take the cover off complete with the drive shaft flange and differential or remove the flange first, I removed the flange first by screwing two longer bolts back through the flange so they pressed against the housing and tightened them gently and evenly (the bolts I used were actually the two from the top suspension arms that separate the strut). I screwed them against the ribbed section for extra strength, you don't want to be cracking the front cover. The flange has got a circlip on the back that's why it resists being pulled out.
Once the 10 bolts are removed you can then prise of the front cover, this needs to be done gently and evenly, you can tap it out if you can get a hammer on the bits sticking out, there isn't a gasket just an o-ring round the side. What I will say is the two bottom bolts are obstructed by the subframe so I jacked up the gearbox slightly so it lifted up on the engine mounts, probably no the right way but it only has to go a couple of centimetres. There isn't a lot of clearance so I used a short spline bit with a ring spanner taped to it to undo some of them, others you can get at with a ratchet. As you take the cover off a fair bit of oil will come out unless you have drained the box first so but a tray under it to catch it all and after the cover is off the diff simply slides out off the splines from the other driveshaft, there isn't a circlip on that end so it just slides out, there is just enough room to manoeuvre it out form the housing.
Now this is where it gets interesting, I bought a second hand gearbox with the intentions of swapping the complete diff and front cover from the second hand box to mine but when I opened the second hand box the front bearing was completely different, on mine it was a roller bearing but on the SH box it was a taper bearing, I thought it would still fit though and because I was swapping the front cover as well then it should all still work but no matter what I did when the cover bolts were tightened the diff locked up tight. I even swapped the original rear taper bearing onto the SH diff to see if that made any difference but it was still the same. As that was out the window the next stage was contemplating a complete gear box swap (actually it was contemplating buying a new car until the weather got better).
Before I did that though I thought I'd try to get a new front bearing but anticipated this would be practically impossible as the bloke in the vide I linked to said you can't get them from Audi, I went to a place called the bearing mart which advertised over a million bearings in stock but as soon as he saw it he said absolutely no chance. With one final throw of the dice I called into TPS just to see if they could help me out, 2 minutes on the PC and he turned the screen round and said is it number 19 on this diagram ****? £35 and we can get it for 9AM the next morning. Amazing, its a part supplied by Audi and approved agents so don't go out spending £300 on used gearboxes to fix it like I did.
What I will say is changing the bearing isn't a simple as lifting one off and placing the new one on, you need heat and plenty of it. To remove the inner race from the diff I got a bearing puller but it was slightly on the small side but still gripped it enough, I had to tighten it to the point that the bars were bending and it was about to snap the puller and then I got a hammer and chisel and gave the race a clout, luckily it then started to slide off. For the next bit you might want to wait for the missus to go out for the day, to get the new one on I put it in my flat bottomed sandwich toaster for a few minutes to get it hot and then it slid on easily. The outer race is a bit more tricky, it was wedged into the front cover proper tight so I tried putting the cover in the oven for 20 minutes on hot and this worked a treat the race just fell out and the new one went straight back in. A little tip though, clean as much oil off the cover as you can as it doesn't half smoke the house out if you don't and don't forget to remove the plastic oil seal and o-ring first.
There is a shim behind this outer race and apparently you can get them in a few different thicknesses but they only vary by a few thou so I just re-used the shim that was already in it, I presume the new bearing was the same thickness as the old one.
I've read that getting an aluminium housing hot like that can warp and distort it but I didn't have any problems and it all went back together ok. Putting everything back together was just a reversal of taking it apart, the leg is damn heavy though so I ended up splitting it completely apart with the bottom bolt so I just had to hold the spring and shocker part to bolt it back to the top mount and I sat the other half with the disk on on top of the trolley jack to manoeuvre it back into position. The only other thing I struggled with was the stupid heat shield that's sort of wrapped around the subframe, absolute sod to get back into position and it now looks nothing like the shape it was before I took it off. I forgot to mention as well there is another half round heat shield over the top of the driveshaft flange that has 3 allen bolts holding it on, they are a right pig to get to but you can do it just about with a 1/4 ratchet and extension. Finally don't forget to refill the gearbox, because I'm a tightwad I nicked the oil out of the SH gearbox I had and used a hosepipe and a tundish to refill it, the fill point is on the left side of the gearbox just under the slave cylinder.
As for the tools I used, nothing special really, Iv'e got a half decent socket set from Bacho, a decent strong arm/breaker bar, you will need one, some of the bolts are really tight, especially the 6 on the drive shaft, I put these back with a bit of threadlock on them just to make sure. I had to buy a spline set but it was only £23 and will come in handy in the future.
https://www.toolstation.com/draper-tx-star-hexagon-and-spline-mechanics-bit-set/p66212
And a bearing puller, this was only a cheap thing and I did break one of the posts when I first tried it but it did its job for cheap money.
Amazon product
This is the socket set I have, its come in useful many a time recently.
Amazon product
Hope someone finds this useful in the future.
