<heh heh> You're both right, it's the ceramic gas diffuser from the SIP set at the railway. I suppose it's been there for at least 5 years maybe more, and does get a bit abused by some of our volunteers.
From memory, the shroud had been removed to clear it of spatter and replace the tip, when it was dropped and the ceramic diffuser was broken and it's been like it ever since. Not actually too sure how much it might have upset the gas flow for welding purposes but as I'm trying to develop my MIG skills (slowly) I figured I might as well give myself the best chance.
So a couple of pm's and a few phots of the offending part and lo and behold the new ones have arrived.