And just to add to the crowd, here's what I've been finding:

I drove from Bracknell to Dunstable yesterday evening, listening to Radio 4. It was re-tuning correctly for most of the journey, until I started nearing Dunstable, at which point it started switching to a completely different station. Similarly, when I was going home to Reading again, it started re-tuning to BBC Radio Berkshire (the horror!) from about Maidenhead onwards. I don't know if the latter is on one of R4's alternate frequencies, but it's 104.4.

This was all with AF and TA both on or off.

Cheers,