I've recently purchased a commercial drinks carbonator from the US for
use in the UK and, of course, when I tried to connect the UK CO2
bottle to the equipment the male thread on the bottle doesn't fit the
female thread on the gas regulator that comes with the machine. I want
to source a UK supplier who can offer a suitable adaptor to allow me
to make the connection safely!

Most regulators are made so that you can unscrew the nipple that the
nut (that connects to the CO2 bottle) slips on. Look at this picture
and see if your regulator looks similar:

http://www.micromatic.com/draft-keg-beer/regulators-pid-841.html?box=parts#tabcontent

Next read this:
http://news.killerhurtz.co.uk/newwheels/reg.htm

Discovered that the US CO2 connector is very slightly smaller than UK
version - you can screw a UK nut onto a US tank, but ya cant get the
US nut on this reg onto a UK CO2 tank/extinguisher. Fortunately,
unlike some regs, the inlet pipe unscrews quite easily, so I just
replaced the nut with a UK CO2 nut (the large brass lump at the bottom
of the pic above) - you can get these from welding supplies shops.

I will bet that this company can fix you right up:

http://www.adamsgas.co.uk/Default.aspx

If not, try a local welding supply shop.

If you like my solution and want to pay me for it, let me know and I
will post it as your answer.

Ok, I will research whether the company you mention are able to
supplyt he adaptor I'm looking for.  If they do then I would consider
that you have answered my question :)

Contacted Adams and they don't supply fittings or regulators at all :(