Hot water switch really hot / Black soot above switch?!

We live in a flat and don't have central heating. For the last few days, our hot water switch has been getting REALLY hot to touch. It was also going like this last month but then seemed to go okay. We usually turn the boost switch on (see picture below), and leave it on for about half an hour which is just enough hot water to get a shower each.

Turned it on tonight. Went back a bit later and it looks like something has been on fire!!!! The black line above the boost switch is like a sooty material! I turned the switch off immediately and haven't turned it back on. Does anyone have any idea what could have happened and why was it getting really hot? Do you think it would be okay for us to use the switch on the right-hand side (that one hasn't been going hot).



Really appreciate any help!

Thanks!

More likely a bad/loose connection in the switch.

don't use it to you can sort it out.

Loose connection inside the switch and needs tightening locate the fuse on the board that supplies it and isolate till its done.


Edit
Seco wins tonights fastest finger

Not only does it need tightening, but all heat damaged cable & insulation needs trimming back to where there is bright copper wire. Tightening the connections on copper that's been heated is going to result in another high-resistance joint, and it will overheat again.

I would also replace the whole FCU.

This thread is really for the leccie boys.