Duncan,
Just reading this thread now and I'd definitely look into the history of this car as there are a lot of things that don't quite add up:
1) You've mentioned it feels low on power and dies after 5k rpm, yes K03 GTI's do start to run out of pep above 5k but it should still be pulling quite hard at those revs.
2) The fact that there is a discrepancy on your V5.
3) The golf badge on the boot really set my alarm bells off as GTI's categorically don't come with a golf badge whereas the GT 170 which your car is mistaken for on your V5 does. I don't know why anybody would source one and apply it to a GTI.
4) GTI's come with dark tinted headlight lenses. From some of your pics, your's look like those found on a regular golf, i.e not much tint visible, although it could just be the lighting on the photo.
5) Are you certain your Monza II wheels are genuine VW OEM Monzas. It's hard to tell without getting a good closeup pic, but in one of your pics there looks to be a lip only found on some replica Monzas.
I don't want to worry you unnecessarily but I would definitely look into this car in more detail.