M1ABD was not on the map when I looked first thing this morning. A check with www.aprs.fi showed that the last beacon was at 0715 Hours this morning.
This afternoon I drove up the hill and found the computer switched off. I turned the power back on again and it re-started itself.
There must have been a short power cut this morning and the power returned before the computer had a chance to reset itself so that it would re-start when the power came back on.
All is working normally again (touch wood).