Today, the builder finally fixed my hearing expansion vessel!
Just before Christmas, the expansion vessel fell off the wall and has since sat where it landed on the hot water tank!
It turns out that the expansion vessel was faulty which meant that instead of just having the water in created by the cold water expanding as it heated. It was full to the brim with water!
The plumber that the builder sent was a really nice bloke - and explained how it works and the fault. The vessel full of water weighed 44lb which is how it came off the wall! I'm just grateful it didn't damage the water tank it landed on!!