After bashing, banging, heating and tapping and buying and expensive box spanner, I finally got what I thought would be a heavily encrusted "short reach" immersion heater out, only to find that it was a long reach, in better nick than I thought. Got a coating of green stuff though!
Anyway, I'm guessing it's about 10 years old? For one, it says "made in Great Britain" (Satchwell Controls, Slough) - and nothing is made here these days, and the other thing is that it is measured in degrees F.