Problem solved.
That was a good tip about fitting a pipe to test pressure coming in.
The problem was the isolating valve on the cold water inlet fitting. It had only ever been turned 1/4 on since installation because it was sticky and because the manual suggests that it only needs a 1/4 turn...
Thanks for your interest
I replaced the stop cock for a new one when I did the water main.
All the hot taps are now very slow.
Cold water pressure seems fine.
I assume that there is no hot water pump in this boiler and that it relies on mains pressure. I suspect a blockage of some kind but...
Problem: Turn on hot tap and initial burst of water which immediately slows to a trickle.
Temp. is OK, cold water pressure fine. Have cleaned inlet filter, no difference.
Problem occured shortly after new water main installed as a replacement for original lead and steel pipe.
Any ideas? Help...