Is the pilot flame actually enveloping the thermocouple when it's
alight?

This boiler is a fanned flue unit with permanent pilot light, a number of
manufacturers had this designs (SD, Vokera, Pott. Puma...).
It is essential to check the fan still operates at low revs when the
boiler is idle. It is possible to get fans which stick (but are otherwise
fine at full power) eventually the oxygen gets consumed in the combustion
chamber and the pilot goes out. I used to think there ought to be enough
incidental draughts to keep the pilot going but that is not the case.
Many thanks for your responses - I tried out the following sequence
and managed to clear the fault:
1) tested continuity on the overheat stat and that seemed fine
2) although the pilot light was visibly lit through the window it was
not possible to confirm that the pilot light enveloped the
thermocouple. Removed the front of the combustion chamber and cleaned
it all out.
3) the pilot flight was actually very small - cleaning out the chamber
must have freed some kind of blockage because next time the flame was
much larger and played directly onto the thermocouple!
4) success!
5) only issue now is that the replacement thermocouple is only a push
fit at the pilot light end and does not clip/screw into place -
currently held up with a bulldog clip!
6) This has worked ok for a couple of days and so may be ok - I still
have the original thermocouple and so may well put that back again if
the new one pops out.
All is well and a hot shower is most welcome after heavy landscaping
in the garden!

Thanks again to all!