The part I needed arrived yesterday, but it was too wet to be outside test fitting the car, so I have had a go at it tonight.
To make things simple I ran a power supply from the fuse box to the aerial, which powered it up just fine.
I then tested the aerial but it picked up no stations, so I then earthed the aerial with some electrical wire wrapped around the base and clamped in place with the fitted clamp. I ran this lead to a bolt on the chassis and tested it again.
This time I picked up a few stations, but the quality was poor with the aerial sat on the dashboard. Obviously this wouldn't be any better in the glove box area, so that plan is out.
Putting the aerial on the bonnet and re-tuning made a huge difference. Many more stations and very clear audio, so this is now going to be a bigger job than planned as I will have to remove the old aerial and roof mount this one.
If anyone has any tips on removing the roof liner, it would be appreciated.