Firstly, are they up to date with wormers etc as this can cause mucky bums...? And even at this late time in the year, fly strike (MAGGOTS!!!) are still causing problems, and nice wet, warm mucky bums are a perfect place for flys to lay eggs which hatch into the flesh eating maggots - If you see any tail wagging in your two, you may have a problem already. Ensure they are up to date with fly repellent type sprays too.
So, in all - They need dagging whenever they are dirty (and deal with underlying cause if possible) - It's quite an easy job if you are fit enough and have a decent (sharp!) pair of dagging shears (manual way, only cost £10 or so, no need for electric shears). All you need to do is restrain the lambs and carefully cut off all the dirty wool down to the clean white (assume!) wool below. Go steady at it and you won't cut the lambs, though a slight nick wouldn't hurt them, ensure you get both sides of the tail and the tail and leave them with nice clean bums!
Hope that helps...
