Thanks to Gusbry GTV for this have copied and pasted the bit you might find interesting reading.
After removing the fuel gage central part of the dash, centre console and tracing the looms by hand today, I finally found the blower relay.
It is a 30A (yellow) relay in a block of two (I think the other one is brown) that are screwed to the RHS of the central fuel gauge console inside the LHS of the driver's dashboard (above the clutch pedal) right up at around fuel gauge height.
You can access this by removing the allen-head-secured panel under the steering column (around the bonnet release) next to the panel that comes off to expose the fuse box. In ours, there was a really tight loom in the way, and I had to unscrew one side of the bracket that holds the two relays in place and swing it downwards to get the relay out.
You could see burning on the casing. I swapped it for the one in the main fuse box that controls the heated mirrors. We may have lost those, but the cabin is all nice and toasty now with a working heater!
Remembering how to put it all back together was fun, but there you go!
I just need to source another relay for the mirrors now!
I just wanted to post this to save others the anguish of what I've been through. I am not a mechanic, an auto electrician and I'm certainly not an "enthusiastic" amateur! I am a tightwad though, and I didn't want to pay a garage many hours labour retracing all the things I'd done.
Hopefully, if you get the same fault, this is one check you can do without having to destroy all of your dashboard like me! Note, this was due to lack of power to the fan altogether - i.e. not working at all, so those who have fans that work in some positions and not others - I would expect this to be something different.
One thing I will say - you do need to be a contortionist to get to and see these relays. I had to sit on the open door sill (facing out) and then lie backwards across the seat with my head under the dash. Not bad for a fat bloke!
Hope this is good info, and apologies if I've sounded patronising at any point to you experts.
Thanks again to original poster.