I was thinking the same thing, it is a baileys header tank and the cap for these are approx £30. I've done some research and some people say that the golf mk 1 and 2 caps will fit, but not sure what pressure these caps are as you can buy them for £3 on ebay! I have changed the thermostat already, flushed out the radiator and put new antifreeze in.
Filled header tank just below half way, driving reasonable, it still pressurises and expels water. So I've lowered the water to about half an inch above bottom pipe and this seems fine (don't expel water) driving normally (but lots of pressure). Put your foot down and it will empty the tank within a 3 mile caning. Baileys header tank appears fairly new.
Cheers, robbie505