In the spirit of forums, i'm returning to this to enlighten others.
Given the proximity of the diesel to the pump and the pressure exerted by a head of diesel on a brimmed tank, I had reason to suspect the pump seal. I obtained the following parts and tool
Fuel pump lock ring tool T10202 - from Laser Tools - approx £30
Pump Seal - VW Part no V1K0919133D - approx £18 (was in stock)
Pump Lockring - V1K0201375 - approx £8 (ordered next day)
I followed the workshop manual to remove the diesel connectors (2) and the loom connectors (2). With these removed you can use the tool to loosen the lockring, which as an engineer I have to say is a poor design. The tool engagement onto the lockring teeth is about 2mm at best, yet you are provisioned with 110Nm to undo, on a pump that's sitting at about 15 degrees from horizontal. This has skinned knuckles potential written all over it.
Anyway, with the lockring removed, you can lift the sealing flange of the pump, and slip the old seal off over the head of the pump and vice versa for the new one. this does not require the pump to be taken out, simply the flange lifting. the manual conflicts with itself about leaving the seal dry or lubricating it with diesel over the space of 3 lines. I opted to lubricate the seal on the pump mating face in order to allow it to slide rather than bind during assembly clamping.
I torqued the new ring to 110Nm and the arrow on the lock ring was nowhere near the marker on the pump, so I used a breaker bar instead to get the remaining 15-20 degrees of rotation such that they aligned. Reassembled and the end result is the leak is gone.
My existing lock ring, and indeed the half thread on the tank, are both rusty which is amazing considering they are against the belly of the car. The material is a poor choice, and I have to say the pump fitment and retention was far superior on my old 306 HDi. There was flaked rust all around which i was having to vacuum up to stop them dropping into the tank, but easily collected. I suspect this rust is what has prized the seal apart and facilitated a leak. I would therefore expect others will encounter a leak in this manner, as nobody has ever touched it on my car. I hope this enlightens others.