David wrote:
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.

I suspect it's a badge-engineered version of something else. Are there any
hints who the OEM is?

Sometimes it can be worth searching any part numbers you can find - they'll
likely be shared with the OEM model, so a search might reveal it.

Theo