Lobster presented the following explanation :
Bingo! Sorted; thanks very much.

Do you know what would have prompted this behaviour? I'm sure that in
the past if ever I've disconnected the power, the unit switched on silently,
asking for the code immediately.

Cold weather, high cranking current, lots of voltage drop and perhaps
an almost flat battery as a result... The radio perhaps thought it had
been disconnected from the battery briefly.

--

Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk