The Medway Handyman wrote:

IIRC the old 1/2" copper pipe is so close to 15mm that new fittings will go
onto old pipes, but I'm not sure about the old 3/4" copper.

Will a 22mm compression fit or do I need an adaptor?

You need an adaptor for soldered, or for compression, get some 22mm-3/4
olives.
These can be used in normal 22mm compression fittings, but are fatter at
one end to allow 3/4 pipe to be sealed well in the larger 22mm fitting.
Only available at decent plumber merchants, 50p each iirc, which seems a
lot for an olive, but remarkably cheap compared to a proper 22 to 3/4
compression fitting.
Alan.
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