Funny you mention taps, my bath hot tap 'head valve' refused this
morning to open to maximum. I get a dribble. Now, off to B&Q to get
another head (standard replacement bit inside - the last one didn't last
long).

I had the same problem (it was only going about a quartyer turn) - stripped
it and re-assembled - job fixed it. I believe the problem is due to water
getting past the first "O" Ring and then causing it to hydraulically lock
due to the water being incompressable.

To strip - remove the circlip on the shaft and press the spindle down to
remove the spindle from the body. Re-assemble with a bit of lubrication.