In article .com,
wrote:
Is there some sort of special fitting like a tap fitting, which will do
the job? A tap fitting won't do it because it has a short spigot, which
I suppose I could file off.

You just want a 15mm compression x 1/2 FI thread fitting like
http://www.screwfix.com/app/sfd/cat/...94027&id=12341

--
Tony Bryer SDA UK 'Software to build on' http://www.sda.co.uk
Free SEDBUK boiler database browser http://www.sda.co.uk/qsedbuk.htm