A big thank you to everyone who commented. Following the instructions
in the boiler's manual I managed to remove the water pump. I tried the
local plumbing place and they recommended Parts Center, who ordered a
replacement pump for me. A quick trip back to Part Center when I
noticed the new pump pumped the opposite way, but I was assured that
for that particular boiler the part had been 'enhanced'. So I fitted
the new pump, filled the system, crossed my fingers and switched on.
Wow, hot water and heating.

While I was there I overheard someone discussing adding chemical
additives to the radiator water to prolong boiler and radiator life.
Is there anything in this? It's not a hard water area, the kettle only
has a trace of limescale, plus there is an electronic water descaler
fitted to the water pipe near where it enters the house.