Victory!
Replaced the tensioner, idler, serpentine belt & alternator. Refitted power steering pump & reconnected hoses, aircon pump, electric radiator fan etc.
No warning lights now! Battery charges just fine. Car runs perfectly! All good!
Happy Volvo V70 again, plus about £250 in labor charges saved. Whole fix cost about £200 for the bits.
Also dismantled and repaired the dead HU-850 radio/CD player that has a known series of failures and did a bit of SMD replacement, so the radio/CD now works too...
Basically various bits overheat and fail with time. You can see in the photo some of the PCB damage - I replace the current measurement SMD resistors with bigger ones and the failed regulators on the topside, plus added a heatsink to them (cut down from one I had lying around) attached with thermal pads.
These radios are really good quality and very expensive, but eventually all of them fail like this. The fix took me an hour to work out what to do and then to dismantle, fix, then reassemble. AFAIK there are no videos or even details about how to do this repair on the web, so I should probably write it up as it's a common question on the Volvo forums...

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