I think you're right. I've owned the car since April 2002, so over 20 years now, and up until July this year it has always garaged. It now sits outside in whatever elements are thrown at it, and it's probably not very happy about it But there's another little car in my life (the 1932 Morris Minor) and it requires garaging far more so than the Deuche. I am keeping my eye open for a garage nearby though.

Mine is also outside, and would not start today. Slow to turn over. I have a spare battery and put jump leads across and it started.
So let it run and warm up. and it would start again. But would not start again this evening.
I'll check the terminals, but suspect it needs a battery

Edit

I will check it's charging as well first.

Idling doens't charge very much...

Yeah, I was reving it a little when warming up.

Prior to jump leads today, it was reading 12v. So only half charged.

When started, it was showing 13+ volts, so no issues there.

Have brought battery in to charge

Mine started almost straight away at -4c the other morning. What really helped mine is fitting a battery disconnect switch under the parcel shelf. Its so bloody useful.
Also I've always run a bigger than standard battery in my Deuches. And always Bosch. I think usually the equivalent of fitting an Audi A4.

Also out in the yard in all weather. It never minds.

@Jonathan

Put my battery back in (once I got the frozen doors open), and it started instantly Though the same as you experienced, only on one cylinder.

If you are using it infrequently, you may want to charge the battery up occasionaly.

I also realised, I have an android head unit that is draining the battery slowly.

So install a switch for the mass or a switch for that android thing. The 57 is starting well, but the car is a bit stiff to drive with the cold. The oil in the batteurs is thick so for a while the wheels bang over bad patches in the road. After some kms it's smooth again

Been getting further issues with starting. The battery has been acting like it's gone flat, even though it's at 12.8v. I've been giving it some thought this evening and I think I've figured out the issue. I believe the problem lays with the ignition barrel itself. I know the internal contacts are prone to corrosion and that manifests itself in poor or zero starting. The original barrel was replaced with a second-hand, but refurbished, barrel after similar issues some ten to fifteen years ago. I think that replacement has now failed and there's increased resistance within the unit, so the ignition/starter are not getting the full power they require. It also doesn't help that the car runs via a 123ignition unit, and I've heard they're very voltage sensitive. Any drop from a healthy state and the unit just won't work.

What are the symptoms that make you think that?