Thanks Floral. I've looked at the fencing online and it wouldn't work for us because we have trees on the perimeter of the garden, and structures like a large shed, which everyone else's cat is always on top of so I can't see us keeping a cat in our garden at all, sadly.
It troubles me that the arguments are so polarised here, though I know my initial posts were very pro-cat. I was trying to redress the balance at that point I suppose, as everyone seemed to be very 'well, it just happens'.
It seems like the pro dog view is, keep your cat indoors so my dog can be free to use the garden - or don't get a cat.
And the pro cat view is, keep your dog indoors (or supervise it) so my cat can go where it wants to, or don't have a dog.
Both are desperately unfeasible and it's pointless to argue for either of these scenarios, because realism is a factor that needs to be involved here.
Luckily, as I said before, most cats will actively avoid an area they know to be dangerous, and that will include a dog-inhabited garden normally. Most are quick to leave when they sense danger. A kitten or an elderly or sick cat won't always be so fast/savvy.
And most dogs will probably not kill when they see a cat, anyway (I think?)
So this is thankfully a fairly unusual occurrence in its own right.
I don't think there really is an answer to solve it completely, given that people shouldn't encourage or train their dogs to kill cats, and that people have to accept a certain degree of risk with a roaming animal - not just dogs but roads too.
I wish there were 'cat zones' and 'dog zones' that people could signpost and live in by choice 
I think that would be far too complex though.