Thanks for the suggestions - there was a copper washer on there previously so I assumed it would need one for the new fitting - perhaps not. The union and the brake pipes are both new from New England Morgan Spares so I'd be disappointed if this was a compatibility issue. Definitely a significant leak (so probably more than a thread sealant should be used for). Back to the drawing board for now.
I did notice that in this
diagram on their site there does appear to be a washer - no. 39 but it's not labeled so I'll see if they can shed light on what I should be using here.
