Its unlikely that the gauge is faulty - although not impossible, if the sensor only has one wire on it short it to earth and the gauge should then read hot.

Thanks to all for your replies.

The non-reading temperature gauge pre-dates the cold spell by a considerable time, so it is not connected to that. The heater works fine so it may or may not be the thermostat. I accept that the thermostat is 18 years old but it is typical of this car that most of it is original.

The reason that I am concerned to understand the problem is that MOT time is approaching and last year the CO and HC readings were higher than I would like although, of course, still below the required limits. I wondered if these would be affected by poorly functioning temperature senders. If these are 2 wire systems how could I test whether it is the sender or the gauge. please?

This car has over 200,000 miles on the clock and is still drives beautifully and is mechanically excellent but, unfortunately, is suffering from corrosion on the rear wheel arches which is a worry at the MOT.

Thanks again for your help.

A short update. The sensor is a one wire type. I tried shorting the wire to earth but it was so awkward to access by more than one screwed-up hand that I'm not sure I made contact. Anyway, no change observed on the gauge. I have decided to get my trusty village garage to replace the thermostat and the sender, then if the gauge still doesn't work I'll just leave it.

Thanks again for your collective help.