I bought myself one of these 12 v 1.5W Solar powered Battery Trickle charger the other day as I have been using the S a lot and the battery on the beater had gone flat due to lack of use.
When I bought it it was on offer at

 

Well if you ever want to upgrade to something a little more beefier... let me know.

I have just the thing sat in the back of my garage that will suit you nicely.

70W - A bit big for the S dash mind you.... but does the trick nicely.
Also known to run my fridge to keep beer cold at Le Mans.

 

Got my Dad one of these.
He hardly uses the car nowadays, free bus travel is king!
When he did need the car the battery was flat, but now everytime he goes out to use the car starts 1st time.

 

[70W - A bit big for the S dash mind you.... but does the trick nicely.
Also known to run my fridge to keep beer cold at Le Mans.[/QUOTE]
did you have to start the engine every now and then to help top the battery up ?

 

Got back to the station at 8pm Friday night, having been away all week. Got into the car, turned the key and nothing except a click!
Fortunately I had the jump leads in the back and there was another person in the car park wiling to help. Normally there are very few travellers at that time of night.
I drove home OK and left the charger plugged in. The next day was very sunny and the car started fine and the same on Sunday. I can only think that as the Station car park is in a steeply wooded valley it didn't get enough light to top up the battery during the week.
I've left the car outside the house this week and will check how well it starts at the end of the week.
I suspect that the battery may need replacing but its only about 18 months old which is a little disappointing if it is the battery that's faulty.

 

did you have to start the engine every now and then to help top the battery up ? [/QUOTE]
Nope.
The fridge can run off 2 leisure batteries for about 5 days, without solar help.

The solar panel actually generates well over twice as much power as the fridge uses.

The fridge uses on average about 1.5W per hour
Solar panel is useless at night.
But even in cloudy weather it is producing more than enough juice to keep the fridge running.

BTW - The fridge goes down to -18C which requires about 2.5W per hour