Hi Hammy - I tend to stick with the vet ones, although I buy mine online from vet medic
http://www.vet-medic.co.uk I am not sure about the difference nowadays, as things have progressed, I was under the impression that things you bought from the vets were stronger than from the petshop (like flea control ) - I have never used sherleys, but from a price point of view, I think although it seems cheaper from the petshop you use more for longer (although I am not familiar with the new one dose one from sherleys) so worked out the same in the long run. I use drontal plus on mine. Julie:)