Thanks for the reply. Didn't realise the extra forums existed as I'd been browsing logged out until posting
From what I have read (hesitant to correct the gurus) the 454 does not use a transponder key, just a remote immobiliser/lock which rules out that suggestion (although I have tested both keys anyway, both of which worked before).
There are no error codes stored or pending. My reader is crappy and probably wouldn't tell me about dead ecu though, however since the engine runs fine once started using a jumper cable that would suggest to me that the ECU is not dead, and would therefore store/show DTCs were any generated (bit of an assumption).
The battery starts my other forfour (and a Citroen C2 I made the mistake of buying as a temporary runabout) just fine, but I gave it an extra charge for a couple of hours just to be certain: no change.
I've also tried the key again since running the engine test (just in case it was previously in a bad sensor position/locked up) and no change.
Going to test ignition signal voltage next, see if I can chase down what is happening to the ignition signal.
It just means a) digging out the relevant terminal pin assignments from the wiring diagrams, and b) having a second person on hand to turn the key.
I'm wondering whether my low-charge-state cranking has led to a ignition signal wire burnout (if the starter solenoid was not operating (due to low voltage) might that have let the signal short to ground for an extended period via the un-/under-energised solenoid?).
PS - mods feel free to move this to the appropriate forum with my blessing and thanks (Electrics in General/Engine and exhaust, take your pick) - I wasn't aware of the existence of the member-only forums which are invisible to non-logged in users.