On Monday, 2 November 2020 09:22:00 UTC, Jedzi wrote:
I need to replaceÂ*the hot tap in my bathroom sink.Â* The bathroom is
ancient, 30+ years old

That's 1990, which would be metric (and not at all ancient in relative terms).

If it's pre-1970-something the pipework will be imperial.

1/2" pipe refers to the internal diameter, 15mm pipe is the external diameter. The same olives or solder fittings will probably fit both.

You can use a 22mm fitting with 3/4" pipe, so long as you use a 3/4" olive.
https://www.stevensonplumbing.co.uk/...-3-4-inch.html

The tap end will almost always be either 1/2" BSP or metric small-bore with a hose supplied to got to 1/2" BSP female.

To join two 1/2" BSP females (hose to hose) you need a nipple
https://www.toolstation.com/brass-hexagon-nipple/p59604


Plumbers I've used seem happy with push-fit, either JG Speedfit or Yorkshire Tectite, but I'm not sure if push-fit will work on even 3/4" imperial pipe.

To fix the new tap in the old hole you may need a
https://www.toolstation.com/split-klick-washer/p93139
or a
https://www.bes.co.uk/fix-a-tap-tap-...-orange-22022/

Owain