[Guest]

As you say most likely dry joints on the blue drive with conductivity of the joint varying with temperature.
The place to look first would be on the tube base panel at the base of the tube with a magnifying glass or try probing with a plastic object to see if the fault comes or goes before taking a closer look but be careful of voltages on this panel.
On the tube base you will see 3 transistors or i.c's with a metal heat sink on them and one of these would be the most likely suspect. Each one drives either the red, green or blue gun.

[planetquark]

Colur BLUE is intermittently missing (comes on occasionally and briefly, most likely after cold start). This fault shows without PC connection on the power-on test screen.

Suspect faulty soldering connection (or can electron guns die in instalments )

I would appreciate any helpful hints - where could I get a circuit diagram?