One possible cause is high value start up resistors in the PSU going very
high, this can mean the PSU doesn't start immediately but can be got
going
by a random mains spike.

Here is another clue, sometimes it will startup when I press the
tv button (which switches between the normal aerial channels
and the sky digibix scrat connection, seems like your spike
theory is gaining ground...

Its been established in the flow of posts so far that your PSU is almost
certainly starting normally and the line O/P is very probably doing so as
well, it seems increasingly likely that the auto-greyscale circuit is
detecting an excessively worn CRT and shutting down the cathode drives. On
some sets an incorrectly set first anode can also confuse the g/s circuit.