Yep, turned out to be a square sort of socket affair on the head of my DI vessel with leaky threads.
I've now bought a brand new one and you wouldn't believe the amount of ptfe tape required to stop the threads leaking. It's not too much of a problem 'cos i'm now static DI instead.
I was always told about two and a half turns of tape were sufficient for most threads - I must have at least eight turns on them and they are still weeping.