I think it's some sort of thermal cut-out, but.....

[polo1]

....you might know different? It sits between the output of a cylinder thermostat, and the input of the immerser.
Havn't come across this before - an add-on thermal cut out ( but it's floating in the air and not against the cylinder or immerser)?.

 

[davesparks]

It's a thermal cutout which appears to have broken off from the top of the immersion heater.
You can just see the stud on the base of it, this would screw in to the brass boss.
I would suggest a thermostat with built in safety cutout to get around this, unless you are changing the immersion anyway for other reasons

 

[telectrix]

as above^^^^ . think that cut-out should be mounted in the top of the heater, separate from the stat.

 

[polo1]

Thanks guys, I've done a fair few immersers, but not seen an arrangement like this before. Wasn't possible to get full access on the day (there to quote), but this one is the bottom, off peak immerser which the new homeowner thinks is not working. I'll be back in the next two weeks (with spare thermostats in hand!).

 

[Des 56]

I think there used to be add on thermostat for immersions,it was an Otto air stat if my memory serves me right