Hopefully the last query before I get this job done.

Most of the openings on my new (14-year old) cylinder *appear* to be 22mm compression threads. However, the fact that on two of them I can't insert the pipe very far in (a couple of mm, a bit more with force) suggests they may be something else. Maybe I just need to get some wire wool on the inside of the fittings and clean them out, or maybe I am missing something crucial.

Plan was to use 22mm blanking nuts on the solar in/out, and then chunk a compression nut and solve on a piece of pipe on every other fitting. Googling shows the existence of cylinder unions e.g https://www.bes.co.uk/22-mm-x-1in-fe...-end-feed-6794 - but I can't work out if these are simply a time saving device to make it easier to align everything for soldering, or if there is anything special about the connection.