I don't use the yearly packs as I have fluctuating numbers!! I use mccaskies of stirlings website (can't remember what it is - the ad is in the back of H&H!) Delivery is dead quick.
I bought a few yearly packs of eraquell and equimax at a trade show, it was helpful to have the wormers at home ready for when they were required and it worked out slightly cheaper. The only problem with them is when you have a few horses it is a bit of an outlay to buy them all at once.
Now i buy them online when i need them as I have a few more horses! I recently bought from http://www.wormers.co.uk/acatalog/index.html, it was the cheapest i found without trawling through every website!