Thanks, all. In no particular order:
I’ve ruled out PSU. It’s still quite new, and more than capable of handling the load. In any event, I would have expected collateral damage if it was a surge from the PSU before it got to the gfx card. Similarly, the wiring (and there’s plenty on the circuit more fragile). No storms at the time (or even recently).
I’m pretty sure the monitor was a fault with the monitor. Not a complete failure, but a single red line running straight down the display.
When I say stressing the GPU, I don’t mean past tolerance, I simply mean consistent heavy load.
I’m currently leaning towards a GPU fault - possibly manufacturing, possibly a cap dead, or died. There’s been no moisture got to it, and this seems likely.
Five days in with the replacement (same model, different manufacturer - EVGA old, MSI new), and no problems so far (though the last worked for 8 months, so who knows). The key difference is the old used 8 pin supply, whilst the new uses the reference 6 pin supply, so maybe that made a difference (and yes, reference design is 6 pin supply).
Guess I’ll just have to keep an eye on things and keep my fingers crossed.