Hard to tell - I can't seem to zoom in on the picture so can't work out the details, but there does seem to be a lot more going on than usual. If the separate stat/reset controls the supply to the immersion (e.g. via a contactor elsewhere), it may well function as an overheat trip. Some immersions have the overheat trip built into the thermostat, so can look very similar to the old ones without a trip.  Is it on an unvented system by any chance? It sort of looks like what you might see on a megaflo or similar (in which case there wouldn't be a header tank anyway).

   - Andy.

Thanks Andy.  I have just had a chat with a plumber mate. He seems to think the red button is the thermal cutout. As you say he also confirmed there wasn't a tank in the roof.  

Gary

There seem to be two case mounted  thermostats one with the red button, one with a pre-set temperature dial, wired in series., and then the resistive element, the white ceramic-like thing, that also looks to have a dial adjustment. Now if all of these are in effect in series, so any one can stop the heat then that has an equivalent level of safety to a cut out plus thermostat as per a  modern system and the red button device does indeed act as a 'death or glory' safety cut-off
However, there seems to be too many wires for that, and the thermostat wiring seems to go off page and then the heater wiring comes back so it is not clear what exactly controls power to what.  I presume it is too dated for makers instructions to be available, but a bit of buzzing with the meter may reveal the wiring strategy and if it is vulnerable to a single sensor failure causing overheating which is what we need to worry about.

Mike.

Thanks Mike.  This was a new build in about 2010. The immersion element cables went directly onto the heating element thermostat. They didn't appear to stray up to the cutouts at the top. 

Looking at my picture i didn't notice the 2 holes in the top of the white immersion part. They might be a push button reset or maybe just a couple of rivets.  I have to go back on Thursday to rectify a couple of other things so will have another look.

Gary

This may help.

Thank you Chris

Gary