I've used Solent Welding and Fabrication. http://www.solentwelding.com/contact.htm , off Newgate lane in Fareham. Best price and good result for what I wanted.
Are you sure someone like Z spars can't supply an off the shelf equivalent?
You could try Foulke's Chandlery Barge at Bursledon, they aren't cheap but do have lots of oddball fittings, and the odd scrap mast section.Perhaps something to replace that roller reefing gismo you mention above.
I have just done the same thing on my westerly33. Unfortunately it is an old rolling boom from 1978, althougzh it has been converted to slab reefing. I have just been told there is a fabricator at universal marina on the Hamble river that might be able to make me something! What sort of gooseneck fitting have you broken?