MG5 LR- Heating Parked Up

[RogerWill]

Waiting at a hospital car park last night, put heating on.After 20 mins barely any warmth.Then saw 12V showing on dash.
So in instances like this, how do you get heat into the cab and if you do is the duration limited by the cab battery.
Thank you

 

[Alb]

Put the car into Ready mode.
The heater operates from HV.