You can buy the Marriages Flubenvet on line easily enough - I had some delivered last week (from Farm and Pet Place £10.99 for 20 kilos plus postage of £2.99) since I reckon it's cheaper than driving half way round the county trying to get some.
Ironically one of my cockerels is currently critically ill because of worms - the vet has warned me he might not make it. Given that I gave them all the Marriages Flubenvet at the end of September just after they arrived, it's got my wondering how effective it actually is unless you can be sure it's the only thing they eat all week. Mine have quite a big run so even if they're not free ranging, I think they find plenty of other things to eat too. The vet has also said that they've seen plenty of cases of bad worms in chickens that have been treated with herbal worming so he's certainly very sceptical of herbal wormers, FWIW.
I'm beginning to hate worms! Especially as now I know they're there and there's nothing I can do to the ground to treat it.
H