UK Has anyone tried Ivermectin Spot-on?

Hi all, I was just wondering if anyone here has tried the Beaphar Anti-parasite spot-on treatment for guinea pigs? It and a similar product are the only off-the-shelf ivermectin products I have found in the UK. I was just wondering if anyone has any stories or experiences about them? I followed advice from another member that even though the box says 1 treatment lasts 4 weeks, you should really give the appropriate dosage 3 times 7-10 days apart. This worked great, but now that my pigs are bigger they needed a bigger dose, and it has irritated the skin on the back of one of my piggies' necks. I don't think I'll use it again, since she has obviously been scratching at it. My vet said that he can only give injections, which is why I self-treated, but he also said that if their skin isn't red and there isn't lots of dandruff, or bald bits, that it might be better to leave them be. I was wondering if anyone else had had problems with this brand, since someone who runs a shelter said they'd heard mixed reviews about it?

The vet also suggested the sellotape test I know that's what I thought, but what you do is try and pick up skin flakes or bits of scab from the problem area on a bit of tape, and he said he would look at it under his microscope to determine the problem. This sounds like a really good idea, I was also impressed with his concern. I was just wondering what other people's experiences with mites and vets have been?