kipper wrote:
Things have got worse. Down to a trickle now.

Try back-flushing it.

Drain the tank; tie up the ball-float and run some water out. If you've
got a kitchen mixer tap, you can sometimes flush it by holding a food
bag over the spout & turning on the hot and then the cold mains
(slowly). The high pressure cold water will go up the hot pipe.

Otherwise, you can disconnect the washing machine hoses and connect one
hose between the hot and cold taps. The cold is usually mains. Having
someone watch the tank to see what comes out is an advantage.