I drained the cylinder today by opening all of the hot water taps and shutting of the ball valves in both tanks in loft. If I'm right, this would normally empty just the big expantion tank, but it also emptied the small expantion tank. This must prove what Largerkhan and namsag said about a hole...
Hot water is coming out clean at the moment. I'll drain the cylinder later and see if the level drops in the heating cistern. I think that's the only way I'll know for sure.
My expantion cistern in the loft keeps overflowning. I've replaced the ball valve and it shuts off ok but it still overflows. Is there another explanation to this problem? Any advice would be appreciated thanks.