After various false starts (and weeks of due to the winter lurgi) I've taken the covers off the wiring for the fan assembly.
Some bits look logical, others less so.
I am about to start tracing all the wiring, but it would be far easier (and safer) if I could work from a wiring diagram.
The whole thing isn't helped by my inability to locate the HJA2500 model on the Howdens web site. It is a 90cm curved glass extractor with a single filter and two GU10 240v lights set at the back. This doesn't match the
90cm curved glass extractor on the web site.Reading so far suggests that the "experts" think that the most likely areas of failure are the controls, and the motor rarely fails. However one common reported fault is that there is power going into the fan (some posts say "into and out of" which I take to be power across +ve and -ve terminals) but the fan isn't working.
Next step is to contact Howdens. Sods Law suggests that this is no longer a current model and a drop in replacement will not be available. However the design includes a length of flexible tube connecting the fan to the wall vent so all may not be lost.
If I can get a reasonable diagram drawn up I will post it with photographs to see if my drawing looks reasonable.
Beyond a certain point, though, time and effort at DIY becomes counter productive as I have an awfully long list of other things to be getting on with.
Cheers
Dave R