2014 L405 LWB 4.4 V8 Autobiography @ 70k miles.

When turning off, dash turns off immediately but engine takes maybe 10 seconds to turn off and makes a slight wobble as it turns off.
This has been happening for a while.
Most recently, has now begun struggling to start, and worse, once starts, 1/5 times, will stall itself (auto, strange eh) after maybe 5 seconds.
Once running, runs great, no lights on the dash and no codes on OBD (surprisingly), seems to have full power.

Suspected sticky throttle body, but have changed for a brand new genuine one and no difference....
Any ideas? Would be greatly appreciated; pickling my brain!

What engine?
If TDV8/SDV8, then check the volume control valve on the high pressure fuel pump.
If AJ133 V8NA or V8SC, then check the purge control valve.
Also check the engine power relay(s) - where they are is engine dependent but you can check voltage at the ECM at engine on and off to see if it switches as expected.

It could also be the DC-DC converter in the ECM having an internal defect and supplying current after the shutdown command as well as taking excessive time to recharge the capacitors on startup.
I have seen this once in the life of the L405 - symptoms were very similar.
Whichever engine, you need to establish an electrical 'known good' baseline - tedious but essential. check every ground connection, every permanent & switched battery positive connection to the ECM for volt drop and mutual shorts - including switched outputs and sensor voltage outputs.

A defective throttle body would give airflow correlation & boost pressure errors as well as 'commanded position not reachable'.

4.4 V8, have amended original post also.

thinking your right on the high pressure fuel pump, is the volume control value a removable part of the pump?
just seeing that the whole thing is alot of money.
also saw a post saying too much engine oil can do the same, any merit in this?

Engine oil level would give you a dash warning, also smoke and potentially an issue with throttle response. the VCV isn't available from LR as a separate item (due to the likelihood of fuel system contamination - take care if you change it) but is available from places like PF Jones. But - you need to confirm it is the cause first, check fuel rail (HP) pressures during startup & shutdown, also check LP pressures and I'd recommend for completeness, checking ECM supply & output voltages.
Diagnosing issues isn't something you can successfully shortcut very often - you might get lucky, but you could go in completely the wrong direction too, I would never recommend playing 'parts darts', it's a sign of a sick mind

Really good points, appreciate it, im thinking so too. How would you monitor fuel pressures and where would i find what they should be to confirm diagnosis? Cheers for all your help.

Monitor via live values on diagnostics - has fuel consumption increased? It could also be a weepy injector, usually this causes a bit of grey smoke but this time of year you get steam as the condensation boils off anyway

Thank you, will get stuck into it. Cheers for your help, sincerely appreciated.