I went up there this afternoon to suss it out and take photographs for the person who designed the lid. I wasn't able to effect a temporary fix as I didn't have the right tools. (Your helpful post immediately above popped up just as I got back off the fell Sam.) I'll try and go back up over the next few days and sort something that'll do for the short term, time permitting.
Meanwhile - fellow cavers - there may be something to be said for using other entrances to the system meanwhile, at least until a post appears on here to let folk know the securing pin for the lid is attached once again (so it can't fall down the shaft).
A permanent fix will not happen overnight as there is some engineering needed. But the CPC is aware now and on the case, thanks to Sam. (The chain and R clip are getting ready for replacement as well; see picture.)