I took the back cover, located in the fridge behind the two bottom drawers (just 4 cross headed screws), and found the drain below the fan and one of the cool air vents frozen solid.
I defrosted the lines using the wifes hair dryer, don't tell her. Fitted everything back together, a little harder to refit than to take out.
Fridge has returned to operating normally.
If this re-occurs, I may have to take you up on the sensor offer. Thanx for the info though.
Mark Hampson., August 2008