Further update

Tried again but with heat gun (1200W) instead of hair dryer.

This easily got it beyond 125C but moved it further away and gave it 135C for the best part of 10 minutes.

Now working again.

I guess when it’s left in standby for a while the temperature of the SoC will cycle with the central heating and at this time of year the overnight temperature could be quite low. I wonder if this is what causes the problem with a SoC where the die is starting to come away from the substrate ?