I have Soays and lambed end of May last year. Fly strike immediately after birth was a real problem - maybe not dupridingly given the bloody gunk on their backside. Two ewes with twins got struck, one ewe (but not her lambs), and on the other set of twins both lambs (but not their dam) - in both probably on day 2 or 3. Horrible, but they all recovered well.
This year my lot decided to get started a whole 7 weeks earlier!