Thanx for the links, all, big help, also found this on Matey Matey, just incase anyone else is reading this with queries. Pretty basic, but at least I understand a bit more now:
Mk2 GTi:
If you have an early 8v you can fit 256mm discs or 280mm discs but it is more involved than with the later models as the caliper carriers form part of the hub. This prevents a spacer being used or a different caliper carrier. You will need to get a pair of 16v hubs, caliper carriers, and calipers (£200 from a breakers) and bolt these on. You will also have to change the ball joints as the earlier cars came with a slightly larger ball joint which will not fit in the later hubs. The hubs and other parts can be found on Golf/Corrado 16vs (post 88), Mk 3 Passats with either the 8v or 16v engine, and anything with a G60 engine. If you want 256mm discs then you must use the caliper spacer brackets from the 16v but if you want larger 280mm discs then you need the G60 brackets. These are harder to come by as G60s are rarer but you can get them from Autocavan for £90 a pair. These brackets fit on the 16v set up so you do not need the entire G60 set up. If you fit the 16v/G60 brakes then you will also need to fit the later 22mm master cylinder otherwise pedal travel will be excessive due to the larger caliper piston. You might want the larger 16v servo but this is not essential. Both master cylinder and servo are bolt on mods. Whilst the bits are off the car I would recommend replacing the wheel bearings at £15 a side. This is cheap compared to the cost/hassle of taking everything apart later when you discover the bearings are knackered.
http://www.matey-matey.com/brake_conversion.shtml has quite a bit of info on mk1 and mk2 models
