Mystery solved: After a lot of pushing, the plumber changed the 'spaghetti junction' of pipe work (which - for the record had worked for the last 51 years according to the previous owner) and alas we now have properly functioning heating. The plumber summised that as he had added bigger rads all round the house (well, 9 of 11...not that we ever asked for bigger rads) that the system couldn't handle it in its current state and as a result he changed all the pipe work under the floorboards free of charge (on the basis that it worked when he came in, but didn't after said 'upgrade'). I think he realised quite early on the consequence of putting in bigger rads but was reluctant to admit it as it meant him changing the pipe work to get it working again. Tried blaming it on the boiler size, to bg's installation to the system...just stopped short of blaming it on a poltergeist. Anyway it's fixed now after a wholly stressful 3 week plumbing bonanza.