Hi Sally,
Yep you're right ITH, different drugs target at different stages. The two main ones are Triclabendazole based drugs and closantel based drugs (although there are others). Triclabendazole kills the very young fluke, so it is a better drug to go with as long as you don't have resistant fluke on your land. There are cases of triclabendazole resistance in the UK but without knowing your particular fluke it is still a better drug than the closantel based ones which doesn't kill the young fluke.
If they aren't needing a new dose of wormer then go with just a flukicide. If this is your first time fluking I'd suggest you do a FEC before you dose them and then one 3 weeks after. Your vet will be able to do a faecal egg count reduction test on this, which will tell you if your flukicide is working.
Hope that helps
Dans
because she can finally contribute on something :-D