I have cured this problem
It is clearly caused by a problem with the box. I worked out that two of the muliplex groups were being tuned to the Soth East/Kent channels, even though I have the antenna directed to London, and selected that on set up
I had to drop the signal level down until the Kent Tx was not detectable and then after a retune, all my channels are the full strength London ones
FYI, not having an attenuator to hand, I improvised by using a length of metal (actually a small Allen Key) to connect the two inners of the aerial plug/Socket and connected the case/earth using a bit of metal resting on the two connectors
The dropped the signal enough to loose the Kent one, but provide a strong signal for London Tx