Large, angular rocks (3-7 cm across) will discourage digging. Crumpled up chicken wire spread over the beds will keep them out for fear of catching their paws. Both will be inconspicuous after the plants grow.
Rue (Ruta graveolens) will repel cats. One plant will protect an area 3 meters across, in full growth.

I have two Catwatch ultrasonic cat scarers which work well for me. They are activated on movement. It is best to use rechargeable batteries in them and buy a couple so that you don't have to wait for one to recharge. This brand seems to have better reviews than others.

It also helps keep them away from the birds, without causing other harm.

Thanks for your suggestions - I've also just read about putting down containers of Jeyes fluid, cats hate the smell and also putting a few drops of Tea Tree oil on used tea bags and scattering in the borders. Armed with all this I should be successful!

Plant petunias!

I'll have to try that, Faery, though it will only work in the winter, here.