Flubenvet premixed pellets ~ use by date

My bag of marriages pre medicated flubenvet pellets has arrived today. The last time I used these, I had 5 chickens and there was no wastage by end of the 7 days.

I'm down to 3 chickens at the minute, and I know they wont use the full bag in 7 days this time. It states on the bag:

Best before/vitamins and minerals present until 05.05.2013.

Will I be able to use the surplus at the end of June for next treatment, or will it not be effective? Beginning of May would be too quick to worm again I think

Thanks 

That's why I switched to mixing in the Flubenvet myself because the use by on the premixed pellets was so short if you only have a few hens. Also mine weren't keen on the pellets used for the premix (it took 3 days of starvation to get them finally to eat the pellets in captivity after being used to free ranging!)