hazard

Hiya, thank you for reading this :-) I'm at the point of needing to confirm with my electrician if we want a 2.5mm ring or 4mm radial circuits putting in. I'm leaning towards the 4mm radials. The only thing I'm not sure about is whether (where there is the option to have either) opting to have...

Title says it all, really. There is a small grey plastic unit under the fuse box, as per attached photos. It is very warm and there is some blackening behind it on the wood panel it's fitted upon.What might this unit be and what should I do about it? I've no idea how long it's been like this...