Thank you for your reply.
The fault only occurs when watching a programme through the box. TV is fine.
Things we have tried to date that have lead us down to there being a faulty box:
rebooting the box numerous times
Swapping the lead with that used for the box in the other room where there is no fault
Swapped the box with that from the other room
Checked that connections coming into the house are not loose
Would welcome suggestions for something else we could try please?