OK I mis-understood, mine does have a thermocouple but I didn't see anything
on the HX pipes, I'll check again sometime in case I missed it.

The flue is jointed such that the whole unit will slide out, but the joint
set back several inches from the wall bracket. The HX flue is tight into
this bracket - no scope to lift & pull. Maybe with brute force it would
give, but I'll go for pumping descaler thru the lot. Interesting to see what
colour the residue will be!.

BTW the model in my series with a thermostat was called Trent.

DIYSOS