Small update, in line with most of the audio building for the GTI, it's not gone well when trying to upgrade the stereo. I've sorted the wiring properly, got everything I need but I have an issue that the cage for the Pioneer stereo I have is a 113mm job and the fascia plate I have needs 103mm cages! Also, I can't seem to get any radio signal but this was also apparent with the Kenwood unit I had so I believe the stubby aerial I have may be to blame. Going to try and get hold of a stock aerial and test that out, otherwise it could point to a problem with the wiring from the dashboard back (which I don't want to deal with when selling the car!).
Still have this horrible sense that I've made the wrong choice by trading up to a Golf R but it's too late now, the new machine is sat with me waiting to become road legal (tax and insured before anyone thinks I've got some sort of monster). Had the car not even a week and already looking at suspension and brake upgrades ready for stage 1 tuning. I have a serious problem and I think I need professional help... I blame Hurdy.