Hi Louise,
I think maybe you could have a bigger problem lurking here than your neighbour's cat. If a cat can get in easily, so can other, bigger, more threatening predators, especially foxes. I'm afraid the only answer to this that will give you peace of mind is a run that is big enough for them to be happy in when not being supervised out in the garden, (minimum of 2 square metres of floor space each) and is secure enough to give you peace of mind that you wont come back one day and find a fox has killed them. There are several horror stories about foxes on here and other poultry forums, it does happen, and even if you think your area is free of foxes or possibly loose dogs, you can never be sure thery are safe unless you protect them. I think its your responsibility to do this, and although there are several solutions to deterring one particular cat, (try lying in wait with a hosepipe or large water pistol) it's not the whole answer. It's just in a cat's nature to explore, chase, and possibly harass, if not actually kill, your neighbour can't keep her cat in the house for ever, and there will be other cats around anyway......