Had the AA out to our FL2 as she wouldn't start this morning. Yesterday she wouldn't start so charged the battery over night and she started first thing this morning. 10 minutes later she again wouldn't start, not even with a jump and no lights on the dash.

AA can't get her going and reckons the immobiliser and engine management system aren't talking to each other... I'm in the process of trying a Hard Reset, so disconnect battery - hold terminals together for 20secs and wait 20 minutes before putting it all back together and trying again.

Anyone got any ideas please?

Unlikely to be the immobiliser, as the security systems seldom give trouble.
When you say it's not starting, do you mean it's turning over and not firing, or not turning over? Hyundai Ioniq 5 Ultimate. The family car.
2009 Rimini Red SE TD4. Gone.
2006 Tonga Green i6 HSE. Gone.
Audi A5 convertible, my daily driver.
1972 Hillman Avenger GT, the project.

Everything was dead, no lights on the dash. Spoke to a friend who suggested a Hard Reset - disconnect battery leads and put them together for 20secs and then leave 20mins. Put them back on and bingo she started!

Glad it's working. The failing battery could have upset something. At least you know what to do next time. Hyundai Ioniq 5 Ultimate. The family car.
2009 Rimini Red SE TD4. Gone.
2006 Tonga Green i6 HSE. Gone.
Audi A5 convertible, my daily driver.
1972 Hillman Avenger GT, the project.