Thanks for all your replies so far. I have considered pulse chargers such as the CTek and others however, I don't have a mains socket outside near the front driveway. I would have to have a trailing mains lead from the front door and that wouldn't be ideal for a long-term solution. The CTek ones are also quite costly too.

I read many positive reviews about using a solar charger to keep a car battery trickle charged and prevent it from going flat due to long periods of not being used.

There are some good reviews for the Maplin's solar charger but it is only a few watts.

Another one I saw on Amazon/eBay looks like a folding briefcase and is about 4W.

I decided to look at a 10W panel as I assumed this would produce an adequate output with reduced sunlight in winter.

After reading some of the replies to my original post, I'm not so sure now?

At present the battery is completely flat. Is it likely that leaving a solar panel connected for a few weeks would restore the charge? The solar panel would be placed inside the windscreen or rear window.

I probably intend to also connect a desulphator to the battery with the solar panel.