As a side issue, I have a similar sized older immersion heater switch. It's not got the complexities of this one, just on/off and neon but I was always a bit surprised by the relatively small proportions. It didn't look as though it could carry the current level but worked without difficulty for a couple of decades after I bought the house. I decided eventually to replace it with a modern fitting ie on a standard 8cm square box. To my surprise this switch overheated [no pun intended] and had to be replaced with a new one. No problems since then but for a while, I had the original switch back in place again to "hold the Fort". I was convinced that a modern approved chunkier switch was bound to be entirely reliable but maybe this was just a "one off"
Dave W