Ordered from Fleabay, fitted and one new O ring later that pipe seemed ok.
But there’s still a leak!
Two dye packs still wouldn’t show the leak up but you could hear it literally pi$$ing out when nitrogen was passed through.
Finally traced that leak to the other pipe (ending 741 C in the original post) but it appears to be the O ring on the connection to the compressor!
Out came the O ring box and the damn fitting O ring is well above the size of the “standard” multi box.
Don’t suppose anyone knows where I can get this O ring without it costing me an arm or a leg?
Approx diameter of required o ring is 30mm
TIA