This could be a result of Freeview moving the default positions of SD and HD broadcasts.

Perhaps you live in a marginal signal area, and prior to the retune were viewing SD versions of BBC1 and 2 in channel positions 1 and 2 of your TV's freeview tuner. 

Since the retune, the default channels 1 and 2 may now be the HD versions of BBC1 and BBC2, which your tuner is unable to receive due to the weak signal.

The SD BBC signals are still being broadcast but may now be stored elsewhere on your TV tuner - have a look at channel 800 and 801, for starters.

Thanks for your reply Guy, I'm sure you're correct, Freeview have re-arranged the channels & hence the problem.

Fortunately I can receive the BBC HD channels no problem, but in our area the HD channels do not broadcast 100pc of the programmes. IE for the news you have to switch back to the SD channel.

Since posting I have tried bypassing the Panasonic Freeview & plugging the aerial lead direct to the TV. I can now receive the SD channels, but reception is very poor & blocking up....

It may also be worth checking that your TV is picking up the channels from the correct TV mast (i.e the one your aerial is pointing at).  In my area I get a stronger TV signal from the Oxford rather than Sandy Heath TV mast, although they are in different directions!  On one of my TVs I have to manually tune within each multiplex/frequency to ensure that I receive the correct (slightly higher strength) channels from the Oxford TV transmitter, rather than Sandy Heath.