I think we are in the realms of repairers breaking lenses for parts Chris. I upgraded from my 300f 2.8 is to a 400DO IS ii for exactly this reason. The lens no longer supported by canon.
Chris they are expensive items and as far as I can tell damn well made ,incredibly reliable tools,so it's pure and simply a gamble without support. I know my luck mate when a fellow member tells you he's just had the last switch plate in EU for my lens,that was what made me change. You might be able to find a repair guy I'd look there before spending my money . Chris they are still fabulous tools to make an image with ie the mark one IS superteles . Sure I tweaked the tool to suit what I do in the upgrade but I wouldn't have changed if I was sure I could get it fixed. The thought of a 2k paper weight and how to replace,weighed heavy for me.
all the luck
stu
