Any idea how much a genuine Alt would be?

Just curious if anyone knows how much a nissan alternator is for the GTIR?

Camskill do them for £159 +vat so go on the assumption that its nissan so times it by 3 and you will have your answer  

blueprint make one also.

Camskill is blueprint I suspect.
You can get one from any local decent ADL BluePrint autofactors for around £100 to £120 on exchange basis & with 12months warranty, new from a dealer is £300+ pending how friendly you are with your local Nissan parts counter.
same alternator as on w10/w11 so plenty of options on possible newer second hand units & support from local auto electrical shop for rebuild if beyond your handling of any repair/rebuild.

Yeah i saw that it was used on a few other vehicles wich is good news for us like you say