The GTi rear exhaust comes in 2 sizes of pipe 60mm or 65mm.
Mine was 65mm and my GT TDI dia was 55mm
so i got this company on ebay
http://www.ebay.co.uk/itm/220751479722to make me a 56mm and 66mm dia pipe and to make it 5" long
they also supplied the 2 exhaust clamps in the price.
I also got a tube of exhaust paste in Halfords £3.99
when you come to cut your old back box off the car you will find
on the pipe 6 dimples in the pipe like this below
: : :just cut the middle 2 dimples.
and when you come to mate your new GTi back box measure at least 4 times
to get right first time you cut it, you'll only have one go
jools