On Mon, 2 Nov 2020 01:21:56 -0800 (PST), Jedzi wrote:

I need to replaceÂ*the hot tap in my bathroom sink.

Why? Leaking gland? Unless that shaft is very worn and tightening the
gland nut and/or repacking the gland might suffice. Dripping, replace
the washer, if the seat isn't cracked.

Mind you getting the cover and/or top of the tap off to do that on a
30+ year old tap could be fun and replacing the whole thing the
"easy" option.

1. Assuming that the old tap is a 1/2 inch tap, do I need to buy a 1/2
inch tap, or will a modern 15m tap be OK?

A modern tap will still be 1/2" BSP for the tap connector.

2. From looking round on the net, a sensible option seems to be to use a flexible hose.

As others have said you might be lucky and the new tap will have a
stem close enough in length to the old to allow it to connect to the
existing pipe work.

Flexible hoses are OK 1/2" BSP tap connector one end (and new fibre
wsher) and the other end I'd go for 15 mm compression.

Due to the existing pipe possibly being 1/2" I wouldn't like to say a
pushfit would work or be reliable. 1/2" is I *think* slightly smaller
in OD that than 15 mm.

1/2" ID. Wall thickness 0.040" x 2 = 0.58" OD = 14.723 mm ie 0.277 mm
or 11 thou overall, murdering up a 15 mm fitting with 15 mm olive
should take that up no problem.

Be aware that a lot of references on the web to 1/2" copper tube
dimensions are for the US not UK...

3/4" pipe and 22 mm compression with 22 mm olive do NOT, BTDTGTTS.
You can get 3/4" olives for 22 mm fittings that does work. 1/2" to 15
mm olives aren't available....

Am I right to think I should buy a 1/2 fitting for the pipe end and,
depending on the answer to Q1, a half inch or 15mm fitting for the other
end?

You will be hard pushed to find a 1/2" pipe end or 15 mm tap end. Try
the otherway round. B-)

3. Should I use a push or compression fitting (bearing in mind I haven't
done this before)?

See above. Push fit are designmed to work on 15 mm OD pipe. If you
have 1/2" pipe (14.723 mm OD) I suspect that might be outside the
range acceptable for push fit. Compresssion can just be murdered up.
B-) You will need two decent spanners for compression.

Oh and probably a basin wrench to get at the back nut to remove the
old tap and fit the new one.

--
Cheers
Dave.