The first thing to do is check your coax sockets, connections, cables and plugs - are they all good quality and sound ?
Which part of the UK are you from ? There was a fairly major re-tune event for the Winter Hill transmitter (which serves the North West) recently that saw a large number of channels moving frequency. These new frequencies are now outside the range picked up by certain types of aerial, which has lead to a lot of people losing channels.
BBC Four HD, BBC News HD, Quest HD (and others) are provided on COM7 / UHF Channel 55 from Winter Hill. This one hasn't moved recently but it has been problematic for me (and others) in the past, when I could only receive it sporadically at different times of the day and often with pixelation. I used to receive it at a much lower signal strength than the others, but upgrading the coax from the loft to the lounge boosted the strength and seems to have solved the problem for me.
Even if you're not on Winter Hill, you might find that your transmitter has had similar re-tune events and I believe that COM7 is transmitted on UHF 55 in other parts of the country too, so your issues might be similar.
If you're not sure which transmitter you're on, put your postcode and house number into the checker on the
Freeview site and hit 'Check', scroll to the bottom of the channel list and click on 'Detailed View'. This will show you your most likely transmitter and the top 'Coverage Prediction' table will also give you a list of UHF channels ('N' in the table) if you want to try a manual tune or check your signal strengths.
Go to the manual tuning option on your Panasonic PVR and try selecting the 'BBC A' frequency from the table, as this is likely to be one of the stronger frequencies you can receive. Then compare it to the 'COM7 HD' frequency from the table - you might find this one to be lower. It's amber in the prediction table for me, hence my previous problems with interference.
The other option for investigating the signal strengths on your Panasonic PVR is via the 'Signal Condition' (or similar) option in the tuning or set-up menus, if it has one. This might be limited to channels you are already tuned into though, hence my suggestion for going down the manual tuning route.