Oh, ok, that's cool - I didn't realise that - but thinking about it, it's obvious that it has to be larger than the 15mm pipe it's surrounding.
Thanks!
I'm wanting to do the same - but I can't find a compression joint that has two 1/2" tap conectors. They do 15mm to 15mm, and 15mm to 1/2", but not 1/2" to 1/2".