Good on yer for taking care with this one
Fluke control is very regional, so you need to ask your local vets and sheep farmers. Here in north Cumbria, until last year we've generally managed to avoid having to fluke any lambs until October - by which time we're down to the last 20% or so. But last year the fluke was never knocked back in the summer, and we had fluke in lambs' livers in August.
So far it's been drier, although still rather cool, this last few weeks, so at the moment we aren't rushing to fluke them. But we probably will use a combined worm-and-fluke drench for the ewes this year, rather than our usual worm-only.
(Please let that last sentence make it go dry and hot so I look stupid
)