Found out the answer myself - I had set up some "guide favourites", and after a retune these all become invalidated so that there are no selections in the favorites which leads to no channels being shown in the EPG - probably the opposite default behaviour to that which would be "user friendly" on a retune.
p.s. Also be careful on an automatic retune that you are picking up the best transmitter - through the wonders of Digital TV the auto tuning picked up a very weak BBC MUX, which was giving just about acceptable pictures but with occasional digital artifacts - it was only when I looked at the signal strength on the "system information" menu that I realised what had happened and did a manual retune using information from http://www.aerialsandtv.com/nationwidetransmitters.html to make sure I got Winter Hill.
