Cold water storage tank started overflowing very slightly and intermittently the other day. Further investigation reveals an enormous ballcock valve - about 1½ in diameter - connected to a 22 mm input. So in my innocence I went along to my nearest plumbers merchants - PTS to get a new one, given that the old one is 40 years old. They'd heard of nothing like it - the biggest they could come up with was a ¾" BSP fitting. Decided in the end to get a bog standard ½" BSP ballcock valve, plus a 22 mm to 15 mm reducer etc. Then I suddenly realised that the hole in the tank was going to be too big. But I thought even making a smaller hole nearby and somehow sealing the old one was still probably my best option. Now it isn't a metal tank. And it isn't a black plastic tank. It's basically grey, not as soft as black plastic, a sort of composite material. What I'm wanting please are suggestions as to how best to seal the old hole - what sort of patch and glue to use, or do I bolt on a patch with sealant.
Thoughts please.
TIA
Keith