In reply to MM6YCJ:
My external connection became very unreliable/intermittent. I opened the case and poked carefully around the socket with an external supply connected and concluded that the one side of the socket was only loosely attached. A careful bit of work with the soldering iron bridged the gap and external power became reliable once more.
HOWEVER, as a result, some of the plugs that used to fit no longer do so and I have had to tinker quite a bit to get others to fit. I have no idea what has caused this but include it as a warning to anyone who may try the same fix.
Thanks for recent summits, 73
The plug problem was a nuisance. I have snipped off a length of wire from the mains power block having a 90 degree plug on the end and attached a pair of Deans connectors (as used on LiPo) to rejoin the wire. I can now break this join, leaving the radio plug in place, and attach a LiPo or other external battery pack instead. This arrangement is proving satisfactory. (This means leaving an exposed male connector, so I cover it with an insulated female when not in use.)