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The update is... it's the power unit. Temperatures are normal and all the other hardware is fine. Unfortunately, with the PC model I have, the power supply can't be upgraded.
Thankfully my techie has a game-ready tower which he'll give me for £40 and will set it all up on Friday. If - as he thinks may be possible - he can string an extra transfer button/box into the setup I can toggle between all 3 PCs. Failing that, I'll have to live with manually reconnecting my old XP machine, which is hardly ever used anyway. TBH I only need it for Photoshop!