Thanks for the replies. Where the nut attaches at the bottom it is completely broken as is part of the plastic box. Looks like it was only held together originally with jointing compound of some sort as very messy. Metal plate appears rusty / very loose. Will try and take a picture of it later...
Not sure whether this is allowed under the regs, but just how competent does one have to be to fit a new one of these, and how easy is it to get the part. Fitting instructions seem straightforward enough and it doesn't interfere with anything major or dangerous from what I can see. Wouldn't mind...