Firstly I know very little (if nothing about Boilers) but wish to learn and eventually be able to maintain my own boiler myself and this seems a good place to learn how to start.

I have a 'Direct Combination Cylinder' boiler and my white meter heating has recently stopped working (the lower cylinder). I believe this may be the thermostat and I wish to change this myself but haven’t the slightest clue how to proceed. Before I buy a thermostat Im going to switch my top and bottom ones to see if this fixes the problem (although I will have to wait overnight to find out). Ive also been told to push the reset button before doing anything on my boiler to see if this fixes it, but I cant find the button. I am confident enough to change the thermostat myself, but I would rather ask advice before attempting it.

Please can someone let me know what I should do and in what order? (obviously switch the power off at the mains first) but after that???... yes I know that little about boilers and also where the reset switch would lilkley be?

I have attached some photos.

Thanks very much for any advice in advance