Would someone else be interested in testing the fan idea ?
The story behind me discovering this is that I had suspected I had an issue with my hard drive. I obtained a 250G drive and ran off that for a while, rebuilding my 500G original. I expect that is was during this that I omitted to reconnect the fan and had a period of stability.
I then decided to fit a 1TB drive and upon refitting this noticed that the fan was not connected. I duly connected it and very quickly got the remote control problems when running with the new drive.
After disconnecting the fan again I've had few problems, I leave the unit on permanently. If ever it is powered off for a period then the remote problems can return.
I do have periods where the picture can become juddery/broken and put this down to possible overheating, a hard reboot tends to resolve this for a good time. Perhaps fitting a fan which is not powered by the Humax unit would be a good idea, or maybe the remote control related components inside my unit need to be warm enough to work reliably. Possibly the fan itself is defective and could be replaced, I don't know yet.