Thanks again for the insight. I have played around with the AVR more today with the following observations......
- I attached a wire from that ground terminal on the rear of the AVR to one of the mains faceplate screws and............Radio Lyca got louder!

So, I canned that idea.
- I then moved the AVR to the original part of the house (room next door) with the centre speaker attached and could not hear any interference. I then put the AVR and centre speaker back into it's intended location in the extension, but this time could not hear any interference. However.........
- As I started to connect each of the 4 speakers which are mounted high up on the walls in the corner of the room, Radio Lyca became louder and louder through the centre speaker (I could hear this as it's the one closest to my ear while I'm plugging the other speaker cables into the back of the AVR). I then plugged / unplugged the various speakers and in summary, it seems that when I only attach one speaker, the interference from the radio station is barely perceptible, but just about there (certainly, not something that would bother me). It's low enough that I clearly didn't pick this up when first playing around with it and only having that centre speaker connected. Disconnecting that centre speaker then attaching any of the others, gives the same effect at those speakers (i.e. you can just about hear the radio station with your ear close to the speaker). However, it seems that re-attaching each speaker has a combined effect to the point that once all five are attached, Radio Lyca is playing fairly strong through all of them (but I hear it most through that centre speaker as it's the closest to me I guess). You have a fairly perceptible step up in volume as you add each speaker in turn to the AVR.
I guess that means that the speaker cables are acting as antenna as first considered in this thread. However, I can't help but compare with the old Onkyo that didn't suffer from this as being more robust.
I have those ferrite core / clamps which arrived from Amazon today, but not sure if they're worth bothering with?
Thanks,
Dem.