Brian Porter : The best way to test if 4G is responsible for your problem is simply by temporarily removing the aerial from your sky boxes "aerial input" socket.
If the problem vanishes, then it could be that either 4G is responsible, or that your Sky boxes RF modulator is set on a channel used by a Freeviewtransmitter, unable to advise on the best channel to use as you haven't provided your location, this preferably being in the form of a post code or one from nearby, i.e: a shop / post office.
Thank you MikeP & JB38 for your responses. My location is Crawley Down, (nr East Grinstead) West Sussex. However, I (we) cannot get to the settings for the RF channel output. It's an Amstarad (HD) model number R010.070.91.00P / version 4F3005. Also, disconnecting the RF aerial as suggested solves the problem! So, hopefully all we need do is to change the RF output channel - so help there please. Many thanks to both of you.
Update - RF outputchannel is 35. Have put free 4G filter in loft on aerial input side of masthead amp. It has improved things considerably so we're on the right track but the picture is still down in quality a fair bit. Is there a different amplifier for the loft/masthead? Again, many thanks for your help folks.