Oh, the vet did apologise. They have offered to pay for new mice. Apparently mice are "exotic animals" and as such very hard if not impossible to treat so I could be risking their lives with any treatment that I every use on them. No offer of the £30 the check up or treatment cost to be refunded. I have thankfully been able to find a new vet that I will be using in future, the receptionist on the phone is a qualified veterinary nurse and even she could tell me that the medicine was too strong and that the vet should not have used it.

Getting new mice. Still miss my little babies though
Thanks for all the hugs!