This is something I've never seen anywhere. The blockage only effects the hot supply to the bath/shower thermostatic mixer valve and i first noticed it about 6-7 weeks ago when the flow from the tap gradually slowed to a trickle over a few days until it became useless. I removed the valve to find a yellow fatty substance blocking the filter in the hot inlet to the valve. I thought it a bit strange at the time but just removed it and re-fitted, and all worked well. Thing is it happened again tonight as i noticed the bath filling up slowly when running it for the kids. Removed the valve and here's what i found. The first pic shows the material as i found it on the filter and the second one after removal. On closer inspection it felt a bit gritty so i dried it out and it appears to be like a very fine sand, as shown in the later pics. It is only this tap/valve that has been affected by this blockage and as i have taken a supply (several years ago without any problems until now) from below the bath to feed an outside tap at the rear of the house i can only assume that the check valves have become knackered for some reason and no longer work as they should. In twenty-odd years I've never come across this problem before. Have any of you lot, by any chance?