Don't always trust what your agricultural merchant says, it always pays to do your homework first

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White wormers are used at this time of year to kill the worm species that is most prevalent now which is netmatodirus. Although most worms now do have a resistance to white wormers, this particular worm doesn't. So in order to safe guard to resistance status of the other products (clear, yellow etc), the current advise is to use white now. Then for the remainder of the year use a different product. The same goes for combination products, you don't want to be treating for something that isn't present.
You can get smaller quantities of some of the products, and not too bad a price - I recently bought some Parafend 1L for £20. The shelf life is usually 1-2 years. Panacur you can get in 250ml bottles. Search online if your local shop doesn't have what you need.
It's also handy to have a white wormer for when you do get tapeworm, as only white wormers will kill those.
Have a browse of the SCOPS site -
http://www.scops.org.uk for lots of info on wormers and flukicides etc.
Oh and netmatodirus can do sufficient damage to a lamb before you will see any eggs shed in the poo, so in this case FECs aren't worth while. I treat anything that I see with a dirty bum or loose stools (not atributable to wet grass) immediately with a white wormer, and I find that it clears up immediately. I aim for clean bums on everything all year round, any sheep that is persistently dirty gets the chop. I rarely need to even dag now.