Double dose Strongid P is what I use on my youngster for tapeworm and also did this when he was 6 months. I was advised that the all in one wormers don't work as well on the babies,as they do with older horses apparently.
It is because the ingredient works by being absorbed into the fat and releasing slowly. As youngsters tend to have less fat, they get a spike in blood levels and are more likely to be overdosed. I know that much, but not sure of alternative! Vet is open now anyway so will ring them and see what they say. Cheers for the advice guys