Back from the trip up the hill. It appears that the TNC had become confused and was only showing a busy light. A quick reset of the TNC and it burst back into life again.
I now see that M1ABD-3 and G0ROZ-3 are off the map. www.aprs.fi says that they both last transmitted at about 2330 Hours last night. I don't believe that all three stations went off air at the same time!
I will wait and see if the auto-restart sorts the problem out. If not I am in for a bit of a drive around the County.