OK, I'm back in business! Thanks all.
Lifted a leg of D3 - it checked out OK. Forgot to read the full number before I resoldered it
Tried to lift Q1 but my crappy solder sucker wasn't doing a good job, so I gave up.
So I removed the 555. It came out in several pieces - it was severely cooked.
Added socket and a new 555. Very cautiously turned on - all fine. Added some load - all fine. No components heating up. Up to maximum setting - no problem. Put the oscilloscope onto the output to the load - it was behaving just as I described before. Loooooooooooooooong pulse width - seconds! At minimum setting output 12V is on for some 40% of the pulse. At maximum setting output 12V is on some 95+% of the pulse.
Ran for a while and all seemed fine. No components got even warm to touch. (the 3" dew strap was only drawing about 0.5A @ 12V).
I think it is on too long at the minimum setting. Can anyone suggest, from the circuit, which resister value I need to tweak to have it on for less of the pulse width at minimum setting - perhaps down to output on for around 15-20% of the pulse width?
Thanks again.