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Update - S'Alice, your tip seems to have worked - somewhat, at least.
Not having pixie-sized hands, I struggled to get at the mini-fan on the graphics card (2 tight, spring-loaded screws & a non-removable - to me at least! - lead), but cleared all the clag ok. The GPU Temp is now showing as 58/60C, though still with a 'flame'.
I'll add the next bit here, to keep everything together...
Having restarted, I got the scariest 'Blue Screen of Death' I've ever seen, along with a high-pitched whine...
Powered off, opened up & realised I'd nudged another card, which I reseated. Restart then told me that I needed (apparently) to download a driver for 'Intel(R) 82567V-2 Gigabit Network Connection'.
I did this from Intel's site, then had to "Restart for changes to take effect." At this point I was also shown a problem with the 'Atheros AR5006GS Wireless Network Adapter' which didn't have enough space to run (or similar wording) & I should disable another device.
I thought - "Meh - Restart anyway, might clear it..." It has, but the Atheros device has now disappeared - can anyone tell me what these things are, what I've done & whether it matters or not? Did I just swap one thing for another?
I'm sorry this has dragged on, but genuinely appreciate the time, attention & generous help I've received!
As regards the OP, I've run a couple of YT clips successfully, so fingers crossed... :-))