Well - I managed to get the can off eventually. I persevered after taking off the driveshaft that I replaced a boot on 15 years ago; I could see that I had re-peened it. Quite a neat job if I say so myself, and it is still doing the job it should. I just couldn't remember whether I had fitted a new can last time or not! Evidently not.
So can is off and now I 'just' need to remove the spider with bearings off the drive shaft. Mine do not have a circlip and I do remember from last time that this took a lot of force! There is not enough of an edge for a 3 legged puller (at least not mine) to get a hold. So will be the vice, a large hammer, and some sort of punch to get some purchase on the spider.
Stretching a boot over the whole thing is attractive, but I can't see how the small end of the gaiter (24mm ID) is going to go over the can and retain its original size to do the job it should. It might however, stretch enough to go over the spider and bearings. That would mean 1x PITA job per drive shaft instead of 2x. So I may put off trying to get the spider off until I get one of these cone thingies, or even the hydraulic tool referenced by Miker7; got to be worth that price to give it a try?
There must still be plenty of 7's about with these older driveshafts? What's the future hold; is there a newer design that fits?