Is it possible to trickle charge a car battery in a car from a seperate car battery outside of the car? My garage has no power supply and it is not feasable to install mains electric.

 

Not really. Say you have a full bucket of water and a 1/2 bucket. If you even the water between the buckets you will only have 2 buckets that are 3/4 full.

Same for the batteries, you will never be able to get the battery to full charge as the donor battery will be depleted.

 

Can you mount a solar powered one (sell them in Maplin's) on the roof of the garage and run the cable to the car?

 

I have a solar one from Maplins which works a treat when I keep the car outside, garage roof seems a good option.