Same as Daisy's mum, we treat twice a year, once in spring and once in autumn. Sorry to hear you lost one, if you're not used to Fluke, it can get really bad before you even know about it. We nearly lost a goat to it a few years back, not long after we moved here, we didn't realise it was a fluke area. We were quick enough to save her, but it was very nearly a different story. Your other sheep should be fine, I would probably dose them again in April time just to be on the safe side.
Beth