I have a really old potterton floor standing boiler and think the
boiler stat has gone tits-up.

it's very similar in appearance to a freezer thermostat in that it's a
box with a dial on it with a thin tube leaving from one side. However
it differs in that on the end of the thin copper tube is a bulb some
7" long and about as thick as a pencil. The bulb sits in a cavity
within the heat exchanger.

Is this a fairly standard part? Where can I get one?

I have tried a Google with no luck.

sPoNiX