From a quick look on the web it seems Primatics are still available, complete with foam insulation. I don't know what the pros think of them, but a direct replacement is an option.I’ve read somewhere this evening, that all that’s needed is another tank in the loft (do I still need this as it’s hot water only?) and a feed from the tank to the return and a vent from the flow. Is that’s all that’s needed?
And would I definitely need another tank or can I just use the one I have as it’s only hot water and not mixing DHW and CH?
I’m going down a right rabbit hole researching all this
Does the back boiler give some space heating? If so I guess you want to keep that.
If you change to a conventional cylinder you need a header tank for the primary circuit (as others have said).