Took the side of the bath off to investigate.
Toilet pan end is siliconed and solvent welded. None of that is reusable
20240427_113718 by
rogerzilla, on Flickr
Soil pipe end is the usual ring seal push fit.
20240427_113755 by
rogerzilla, on Flickr
So it's going to have to come out of the soil pipe. Lots of silicone lubricant will be employed. The word is that these often leak if disturbed after many years, so that brings me back to my original question - is there any way to renew the rubber ring seal in these? It's not easy to replace the whole tee due to the bath and basin connections, although there are no further connections above this. People seem to use sealant and/or self-amalgamating tape in the event of leaks - it is all very accessible once the shelf at the bacl of the bath has been ripped out, which it will be.