ivomectin would be my choice on droplet on the back of the exposed skin on the back of the neck about every 4 months will keep them clear of internal and external parasites this would probably be the simplist way for you ,its good stuff orginally produced for cattle but now produced for all animals dogs,cats, birds etc except collie dogs it can kill them ,check with your local animal vet he will measure you some out should cost about 3/4 quid for a small bottle