Hi everyone
Thanks for advice at time of original post.
I have found an answer which works to fitting an reducing valve with an American CGA-580 fitting to a British gas cylinder with a "BS3" fitting to the valve and a "shield" around the valve.
Bit of terminology - what is often called a "bullnose" here in the UK is called a "nipple" in the USA.
Bit of context regarding US and British gas bottles. American gas bottles come with a protective cap which encloses the valve closely, which is about the size of an eggplant/aubergine. Once removed (unscrewed), the valve is completely exposed. British gas cylinders come with a not normally detached "crown" on top of the cylinder who walls form a circular enclosure not much smaller in diameter than the cylinder and about 5inch/125mm high, in the centre of which sits the valve, protected during delivery and when the cylinder is in use - the regulator / reducing valve fitting to the valve by the stem of the bullnose/nipple sticking through a single cut-away exposing the socket, specifically for this purpose.
A CGA-580 nipple/bullnose fits just fine against the BS3 valve fitting seat. A BS3 screwed collar has a smaller bore through it than the diameter of a CGA-580, but has sufficient metal that you can put it in a lathe and bore it out to making a rattling fit on a CGA-580 fitting, no prob. Do make it a rattling fit, as everything is supposed to be concentric, but I have had one fitting only sitting on the seat of the cylinder valve over part of its circumference, with a major gas leak past the rest.
In America you can buy longer nipples, of which 4 to 4~1/2 inch (100 to 110mm, but we are talking of the US here!) is readily available, which will allow and America valve to couple to a British cylinder and BS3 valve fitting through the opening in the protective "crown".
Richard Smith