Thanks for your replies.
I originally thought that the problem was of the usual dry solder
joint type but now I'm not so sure.

Switching the TV off and on using the standby buttontakes less than a
quarter of a second and it always fixes the picture if only for a
little while. I wouldn't expect this if it was a problem with heat
expansion of a circuit board. Also the room temperature doesn't seem
to make any difference. The connector at the neck of the tube still
glows when the tube has gone off.

I don't know too much about electronics, I'm a former electrician so I
have a grasp of the basics but could it be something to do with a part
of a circuit discharging when the picture is switched off? Or voltage
regulation perhaps? I told you I don't know too much electronics!

I had hoped that the fault would be of a type that someone would
recognise as a particular one.
Otherwise I will have to take Jerry G's advice and take it to a
repairer.