Well, for the benefit of anyone in the same situation doing a search - the next phase of the job.
Sump sourced from GSF for £70-ish, who don't sell the sealant goo so a tube of insta-gasket stuff (smelt exactly like the silicon I used to seal around my bath) from my local auto parts
store did the job.
First sump leaked a little around the plug and had a hairline crack around the inset where the level gauge goes, a manufacturing defect judging by the rough alloy on the other side of the crack.
Swapped it for another at GSF with no trouble, found this one also leaks a bit around the sump plug, but more crucially the holes tapped into the sump for the level gauge were about 1/4" shorter than those on the original sump, meaning the level gauge couldnt be bolted in tight. I used a flat and a spring washer on the bolt to make it fit right.
(NB - I'd avoid GSF pattern parts in future. My Citroen owning brethren aren't overly impressed with their generic no-brand kit either. Seat main dealer wanted about £90 for a sump which was presumably the genuine VAG item)
Fresh oil, fresh filter and start it up.
The engine itself sounded fine, and for ten, no fifteen wonderful minutes as I drove home I thought I'd dodged an expensive bullet.
Then the turbo started making a hell of a racket, boost vanished, and the car started making enough smoke to be worthy of a James Bond movie.
Arse.
But at least the rest of the engine appears to have survived.
