Hyperdrug have, for the last two years, been the cheapest but I notice they don't have the 500 in stock currently. The other company I've used on a regular basis is
Chemist4UFor future reference it's always worth remembering to buy in Winter rather than the height of hay fever season as they certainly seem to hike the prices!! That said I can see I'm going to need some more soon so I should take my own advice. The trouble is that the people and dogs in this household have all needed more than usual this season... :)