Thank you very much for the idea Aunt Sally - I'll take a look at those links but it might be that the price of postage is prohibitive.
Our experience here with vets is that they are much cheaper than the UK. For example annual injections for 2 dogs plus worming tabs and scalibor collars which do fleas and lice is about 50 euros (i.e. aout £30). There are no extortionate charges for medicine if they need it - the vet just writes you a prescription which you take to the ordinary pharmacy and you pay the price on the packet, which so far, in our experience has only been a pound or two.
It might be that there is a Spanish equivalent of flubenvet and diatom. Particularly with the worming I am keen not to give anything that would mean discarding eggs for a while.
Dawn