if the problem is simply getting good purchase on the pedal spindle there is a (fairly time-consuming and fiddly) method which might work for you.
Thanks. The collar, assuming that's the plastic thing, is already damaged by the spanner slipping off. So replacing it is a possibility already. One way and another, I've been using SPDs for many years, but I've never felt the need to strip one down, so this is helpful. I even have the tools to do it, just never really tried. The bearings have always felt good. I ought to have done it, but...
It's a basic Stronglight crank and a low-end SPD, so replacing one or both might be easier, but getting them apart while still usable would be far more satisfying. I may try the Plus Gas first. Or even Super Crack (see jokes above about that).
Mind you, you're talking to someone who has broken the jaws on two bench vices trying to separate bike parts.