Thoroughly tested every-which-way with the new starter still on the van, and the old starter on the deck.
Tried the trigger, the main lead, the solenoid and the starter all in isolation and combination and with the van battery and a standalone one...
End result is I have 2 starter motors, old & new, each with a different fault

The old one is most likely the usual commutator/brush being worn out - will try to strip it myself and have a play

The new one is most likely a manufacturer QC issue relating to the braided jumper twixt solenoid and motor internals

A WORD OF WARNING TO ALL (just a statement of unembellished fact):
1. I purchased a new starter on t'bay auction site for about £65 delivered from Germany with 3 years warranty.
2. It don't work right.
3. They expect me to front-up the return postage - to be refunded on acceptance of fault.
4. If it's a manufacturer fault rather than their own (eg 100% of returns, as they just sell-on parts) it might take 30 days to confirm.
5. You have to confirm ridiculous things like odometer reading and chassis number and if you have ABS.
Their suggestion: buy another, it's there in 3 days, yay!

My plan: take this one to a refurb specialist and get it pimped up with more effective connections...

Hopefully back on the road soon...