Motor works are advertising one for £495
That’s not a 30L tank. It’s the standard sized one
eBay ad is finished.
It's just a straight swap, no real pain involved, well except the eye-watering price of the GSA tank. If youre swapping internals over too you can bend the float arm slightly to correct the fuel gauge. some say its best to get a new sealing ring if youre doing this (personally ive reused them several times without issue) and careful not to strip the little threaded studs.
Wow, that's a lot of money.
They go for about 250 Quid here in top condition, but postage might be an issue. I have both, almost never use the big one.