The job has actually been completed and I will post photos, explain what went well - and badly - along with text and diagrams over the next week or so. I have done this before - mostly with modern UV glues - but this time I have experimented a little to find out "the most foolproof method". Please don't let this be one way communication -and I don't claim to have discovered "the method". But perhaps if you restricted contributions to the step I am describing it would make the whole more logical for those who join later.
The candidate for rejuvenation is a Suter Aplanat A No. 4. A nice big extra rapid RR.
This first picture is what the front cell looked like after removal from the mount. This is cheating a bit - but the yellowing is more obvious out of the mount. Apart from the discoloured ring, there were much larger areas of colourless separation well towards the centre. The rear cell was in perfect condition.