Problem solved - quite straightforward really........
The "short journey" was actually only 2.8 miles miles and just before I got home, I was stationary at a junction for a while and the remainder of the stetch is slow, due to kids palying etc, so was taken slowly. When I stopped, the fan was on - which I thought was a little premature. Not wanting to lose my finger tips, I waited until the fan went off and checked the temp of everything under the bonnet, hence my original post as lunchtime today as the radiator was cold; I now realise that the fan had cooled it very quickly and the ambient temperature was also quite cool last night.
However, I drove home from work today (about 1.5 miles) and got the same symptoms, but no fan. Drove back, adding a slightly longer detour (no lingering at junctions) and checked again - all was warm including the radiator and no fan.
Being useless under the bonnet, I've now twigged that the thermostat hadn't had chance to fully close.
In a nutshell............panic over. I hope.
Thanks to all for their contributions.