Have been doing a bit of playing about to see if this was some kind of problem with my TV...
For the first minute or so of the 7800 being on, the picture is fine but then it would begin to intermittently go b+w then to colour again.
So I started playing around with the settings on the TV and adjusted the AFT (Auto Fine Tune) and voila! The 7800's picture stayed in colour - however, the graininess of the picture was now unbearable...
Anyway - my diagnosis is that the 7800 transmits RF on channel 36 as it is switched on and the TV has no problem with this, however, as the 7800 warms up to operating temperature, it seems to slightly alter the RF of the 7800 to somewhere between channel 36 and 37 and the TV doesn't like this.
So - at least I have a theory, but my problems still aren't solved.....