Dont know but have you considered software problem.
Well g*****s, I can hardly believe it but you were spot on. A reset did after some time cure the fault. Here's my take on why....
The customer only has a sky box connected with no aerial connection. After finding out that these sets look for updates two hours after switch off I turned the software update option off. My thought is that the set started cycling to find an update and got caught in a loop of powering up and down eventually upsetting the main proccessor. Resetting effectively stopped the search for software.
I don't know? but a big sigh of relief here.
Thanks again for your input. Really appreciated.
Geoff