I noticed that I had lost a limit screw from my rear mech, went to replace it with something similar and found that the plastic wedge behind is missing too, is this available as a part or is a bodge possible?

🙁

If it is available it will be from Maddisons and very dear.

Get smaller nut and two bolts and put one bolt either side of hole. Will be very trial and error to set up initially I think but will work.

I just used a bit of rawl plug and melted into the hole – crap really but works, bought a ti bolt kit from superstar off ebay for the actual lost screw, seems to have worked fine.

It will be from Fishers not Madison, so if it is available it'll probably take three attempts before you get the right part.