Is there anyway you can isolate it off so that you can temporarily stop water refilling in to the hot water tank? (You could tie up the ball valve in the expansion tank perhaps) If so do that then drain the tank some way. Then re open and allow the system to refill the tank naturally once more. You should be able to here the tank refilling at a reasonable rate. If not there is a partial blockage on the inlet side.
If it refills well then the problem would appear on the outlet side of the hot water tank. It is a matter of tracing back all the way to the tank. If you turn the tap nearest to the tank on in the bathroom is there a reasonable flow? If not then the problem is somewhere between tank and taps. it could be that the outlet of the tank itself is partially blocked. That could either be by some foreign body getting in there or the outlet port itself could be furred up with calcium.
If you can drain the tank as much as possible and tie the ball valve up to stop it refilling.Then undo the pipework where it comes off the tank, you might be able to spot the problem.
If any other questions feel free to pm me.
