Hot water supply has suddenly become very slow

Naf

ThisIsWeird said:

"so I'll be firing some mains pressure back up it with the tap adapters I bought today." Oookkkaaayyy, so you are going to cobble together a length of hose, with a controllable valve on it, leading to a tap adapter that'll fit over the miscreant hot and cold spouts? Ok, absolutely ensure that the gate valves are fully open (ie, all the way, then closed a half-turn - which makes no difference to the flow), especially the one supplying the DHWC.

Yes, this. I printed this plug, expecting to need to restrict the overflow pipe, but it turned out I didn't. 

Did the cold first. The plug adapter I got didn't fit the bath tap, but I got a good enough seal just holding it on. Solved that.

For the hot I hooked the sink hot & cold together, opened the hot all the way and the cold very slightly. Went up to check on the CMS and was surprised nothing was coming from the overflow. Gradually increased the cold and still nothing. I was wondering where it was actually going until I heard the overflow outside and realised to check the normal outlet hole where I could feel warm water flowing in. Not as fast as I would have thought, even once the cold was on full - but it never came out of the overflow unless I restricted the normal outlet hole. Even so, we now have good flow again.

ThisIsWeird

Perfect result - very well done