We’ll the engineer turned up (it was Alan) and swapped the pump head and I’ve got a working boiler again.
The old pump head was stiff to turn so must have stared to seize somehow but it was changed no questions asked, I just hope the new one lasts longer than 6 months this time.
As part of home maintenance, fire your CH up for an hour, at least once, during the summer months. It'll give any moving parts a workout and reveal any defects before they become urgent.
The 'funny smell' customers ask me about when they first put their CH on, around this time of year, is 6 months or so, worth of dust baking on the pipes and radiators