jonesy said:You can get a kind of plastic stuff for sealing leaks that you mix and then blodge all over the cracks; it sets hard forming a waterproof seal. No doubt DIY purists would scorn it, but if it works, who cares! It isn't expensive.
Can you. I shall investigate that for the future. Draining it has cured it though. I think it was the overflow that was weeping caused by the ball valve needing adjusting. As I'm not using it, I just drained it, problem solved.