Yes - though the ballcock is set almost to the bottom and the overflow is
mm
from the top - hence the boiling problem. I would have liked to use a
larger tank but the roof-shape prohibits it. If it had been a problem I
was
going to put another tank in parallel.

OK thanks - the small 4G tanks must stretch further than I thought.

I found a 15G tank ("Titan" PC15R, about £20) on-line. It's 24x17x17 - I'll
go for that and be done with it, setting the float-valve & overflow about
1/4 way down if I can.

Egremont