Sounds it. Once lead acid batteries collapse they collapse really. The recent cold snap is enough to kill nearly dead batteries as well.

Will it jump start? As a last chance you could try, leave the engine running for a good 30-40 mins and see if it still isn’t holding charge.

But, by the sound of things it’s time to just get a new one.

left on charge overnight. Still 11.8v

Yes.

Is it a sealed battery? If not, have you checked the acid? If it's holding at 11.8v it doesn't sound dead. Second the jump start idea, it's worth a try.

I've just had similar. Garage checked and said its OK, get the system checked by dealership (Honda). Honda plugged it in to their computer and straight away said "Replace battery". Combination of little use in 9 months + cooler weather has done for it, I think. 6 years old.

Given the weather I'd say yes - as per the above, cold snap definitely kills nearly dead batteries. 530d is a big old lump to turn over too.

All that said - make sure your hedgehog is ok - have you had any funny behavior from the heater/fan etc? I can't remember what the symptoms are but the 'hedgehog' rectifier has a habit of failing and will flatten the battery.

Edit - actually just checked, it's not a rectifier it's the final stage resistor for the fan/heater.