Considering their ages and that you have had a cat previously hit by a car I wouldn't consider free reign an option but I would look into cat proofing your garden and maybe allowing them to use the garden during the day as well.
There are a lot of companies around who will give you a quote to come in and cat proof the garden for you if you haven't got someone around to do the DIY.
http://www.purrfectfence.co.uk http://protectapet.com -- this one I believe is in Stoke
If you google there are quite a few companies providing this service now and it will give you peace of mind that they have more space to run around but also they are still enclosed and protected, plus in the long run it works out cheaper than the vet bills from fighting