Did this ever get fixed? I have exactly the same problem. The boiler temperature slowly drops, maintained by the heat from the hot water cylinder, which is also distributed around the house radiators (S plan, both valves open), until around 43deg. Hand on pipes confirms the water is being heated and circulating when the boiler is burning. All comments so far seem to address different problems, not this one, so I assume it is not particularly common... Any help welcome.