I haven't seen anymore in the new little tub that I have set the Isopds up in so hoping I may have caught them but from what you have said I wont worry TOO much now. As long as they aren't harmful to the frogs when they go in there. I'm hoping to get my frogs in about four to five weeks time so that should give the isopods and springtails time to sort themselves out.
Thanks for your advice.