I'm interested in this too, Rob. I've not seen it mentioned on the forums, but have you guys looked at broadspeed.com? They were quoting over 3k discount on the RS3 before it was pulled. I used them to buy my B7 A4 in 2005.
I got a bit carried away with the finance functions in Excel but its possible to work out that for my mileage and term, the bubble payment at the end is about 42% of the total cost. Spread the remaining amount over 48 months with an interest rate of 6.8%, throw in my own £5K deposit and it came to about £560 a month for £48k's worth of RS3.
Now that assumes all the optional extras affect the guaranteed final value (which reading this forum seems not to be the case) and £3k of dealer discount. Remove the £3K discount and it rises to £603 a month. If you then assume that the GMV is based on the stock car alone, then it further rises to £670 a month (as you are paying for all your added options over the term).
This is just with a spreadsheet, talking to real dealers and getting a final price will vary a lot. When I bought my last pair of Audis, I went to three different dealer franchises and got three entirely different prices, so it pays to shop around a bit.