My solution was to remove the lever part and rebuild the missing plastic around the holes using a soldering iron. I used the tip of the iron to carve out some 'blobs' of plastic from the bottom of the switch housing and then fused these to the area around the holes to rebuild the worn bits. You need to make sure the holes end up the right size either by running a drill through them or filing. Also you need to avoid protruding bits of plastic that might stop the lever from rotating cleanly on the switch body.
It only took me ten minutes to sort it out and it seems to be working just fine. I'm guessing that others might have the same problem with their switches and this obviously avoids having to buy an expensive replacement. An alternative would be to buy a cheaper passenger side switch or MX5 switch and swap over the levers I guess.
Photos would have been very helpful, but I didn't think of it at the time
