Have a look at scops, this is the most up to date thinking on parasite control.
Nowadays, they recommend managing pasture carefully, using cleaner pastures for lambs, etc, but in order to slow the worms development oc resistance, they recommend worming and turning out on dirty pasture. The wormer can only kill what worms are inside the sheep, excluding any resistant worms. Any resistant worms will then produce eggs that pass out onto the pasture. There will be eggs of worms that were killed by the wormer already on the pasture, so the sheep will take in a load of eggs from these susceptible worms as well if they stay on the dirty pasture. Because the worms have to put 'energy' into being resistant, the ones that aren't will beat them when growing inside the sheep, and over time, there will be a number of worms inside the sheep again, but again will mostly be susceptible to wormer.
If they went on clean pasture, the only eggs they eat are resistant to wormer, so when they grow up inside the sheep, wormers wont work!
Hope that's clearer than the mud i think i made it into, but leaving them where they are is a very sensible option.