Rather than pay another few hundred quid for yet another repair/part, I've just bitten the bullet and booked a new boiler installation for 19th Jan. (Irritatingly, and with no excuse, I'd forgotten the VAT change. Should have had it done last year. Hey ho.)
This will be a Vaillant, which is what Boiler Man David recommended, and with a bit of luck will be more reliable, easier to get serviced, and possibly even more powerful . He is from the same firm that installed our LPG gas fire (which is lovely, and has 'just worked' brilliantly ever since) so I hope will do a good job.