Spoke too soon! I still have problems with this. Tried a cold start (room temperature of about 10 Celsius / 50 Fahrenheit) and voltages were about 10% below spec. A quick blast with the heat gun and voltages were in spec within about 15 seconds and stayed there. Rock solid after this for an hour until I turned it off.

So I'm wondering if there are any other components apart from the caps that could cause this. I desoldered most caps again and tested them for ESR. All good so far. Could it be the feedback that needs warming up?

It's a nice little machine, would really like to get this one going.