Allo, allo René.
12.4v is a bit less than half capacity so you maybe have 12 days before its so low the engine wont start. However, getting the car to start is one thing - it takes far more running than you might think to fully charge the battery. On my boat, it takes more than 6 hours because of course the current flow into the battery falls rapidly as the back emf builds up.
Use some thing like an Optimate. I use a solar panel on my boat as well as a wind generator. But the latter wont look good on the car.
The simplest solution is a swithch on the battery terminal but I dont know whether that would be accessible on a Morgan.