Cleaned everything I could do. The internal black rubber diaphragm looked a bit iffy in places, but hoped would be ok.
Refitted everything, the square on/off dial/tap turned to off position.
Put in 5 litres fuel (wanted to give a good slosh around inside) no leaks so far.
Turned the round tap to on with a spanner, and fuel started coming out of 3 places

Both large side tubes (which connect to the carbs) and the little downward facing hole (not the little side one) all three were leaking petrol out!
DAM.. that's not supposed to happen is it?
Drained the tank (mouthful of fuel - of course!) and took things apart again, Pulled the filter out, it was split a bit (split a LOT now!) and the tap unit apart, and yes, the rubber diaphragm does look worn with some distortion.
So, would I be right in thinking I just need a repair rubber kit for inside (two bits of rubber?) / bit of plastic? bit of metal in between then both?
And what about the filter?
Thanks....
Oh and also.......... Can I test/run the bike by just having, say, washing up liquid bottles with petrol in them, feeding the pipes to the carbs to fire up the bike?
Willa firestorm work via this method? as there are many other pipes connected to the tank?
I'd like to reconnect the carbs and run a test, but as I say, the tank is, at the moment, out of commission (so to speak)