Thanks Denis, that's good news. I won't have access to the other digibox until the weekend so I'll be trying it then.
In the meantime, last night I tried numerous other channels around the same frequency and on various higher frequencies but the only other channel that geve me some very minor breakup (although nothing compared to the breakup I'm experiencing on to Channel 4) was the "BEN" channel (on 12523 H). However, none of the other channels on the 12523 H frequency gave me any problems at all
Unfortunately Channel 4 is the only channel broadcast on 12168 V so there are no other channels for me to try on that frequency
So, the problem really does just seem to be confined to Channel 4 on 12168 V which does make me a little pessimistic that another box will actually resolve the problem but I will try it anyway and report back the results once I've done so.