And there's always the risk you'll need a new cylinder.

Thanks for that, but I think I will leave it, at least for the time being. I have already had one plumber round who concluded it wasn't going to shift without damaging the cylinder.

I guess my best solution for me longer term would be to get the ensuite shower connected to the combi boiler rather than the hot water tank, that shouldn't be too bad a job to do and would give a bit of redundancy in case the immersion heater in the current tank fails.