I'm guessing the experienced farmer will be checking daily, and will spot any fly issues at a hundred paces.
If I remember correctly, you're in West Cumbria, with Shetlands? Not impossible to be getting flystrike in early May, but a lot less likely than further south.
My Shetland ram and wethers stayed with the flock year round, and never any problem with the tup and lambs at all. Silly, giddy, wether hoggs sometimes
but not the tup. They had plenty of space though - stocking was nearly an acre per lambing ewe, and lots of varied terrain, woodland and riverbank as well as pasture and meadow.
My initial reaction was to say that plenty of sheep eat nothing but hay all winter, what's the problem with the tup and companion getting nothing but hay for 10 more days? But maybe you're bothered about escape tunnels and how far a Shetland tup could get in 24 hours.. Or embarrassed to ask your farmer friend to dispense the hay and water?