A few days ago, I had the No Aerial symbol on most channels, so I re-tuned the PVR.
I was surprised to find that, in addition to the normal channels, it stored several German channels (eg RTL, SAT1, etc). I was able to view all these channes perfectly well.
The following day, although the channels were still listed, there was no signal on any of the channels.
Re-tuning the PVR resulted in loss of all the German channels.
Any ideas how to get the German channels back?

What you were experiencing was due to atmospheric conditions, which cause very distant transmitters to travel way beyond their normal coverage areas, (known as DX-TV reception). It mainly happens during times of high pressure.

Chris.