Lexia tells me there is a problem with the power supply relay latch, but swapping the underbonnet panel (BSM) with a known good one made no change; for giggles I proved the faulty van's panel also worked fine in the donor van.
Further, Lexia doesn't talk to the ECU at all. Not a squeak, which says to me it's not getting power.
Desperate for suggestions; I really want to get this sorted ASAP as I've booked a week away from Wednesday.