Guys,
Thanks for your posts.
The 4 round rings which are fitted to the rear box are relatively easy. Mine came as part of a non oem exhaust kit so following your advice l will be careful to check these for condition on a regular basis.
The long one in the middle though is the stuff of nightmares. This one is an original from MB (A1114920082) - my car is a W111. After a bit more research I suspect that the W113 does not have this ring.......and I can understand why. The hooks for it are inside a recess in the floorpan. The recess is just big enough to get the rubber ring in. I found it impossible to apply any leverage to stretch the rubber once it is in the recess and fitted over one hook.
However I will invest in one of the tools suggested (thanks for the link Hans) and try the hot water technique. What are we talking about here - boiling point? I guess these rubber rings are designed for taking heat bearing in mind the application.
Regards
Peter