The 'natural' time for the Shetland tup to go in is Nov 5th, start lambing April fools day-ish. Being mature ewes I can't see you having a problem with conception, and to me, unless you are working away or have some other problem, sponging a primitive sheep is unnecessary. It just complicates matters, when the sheep are perfectly able to get on with reproduction without our interference.
Ours usually lamb within about 2 weeks of each other, in mid April - just in time for the new grass to make them good and milky. We leave our tups in for 4 weeks, and we set aside 4 weeks in April for lambing. We used to leave them in for 6 weeks but occasionally there was one very late lamb, into May. I would rather not have that, and prefer to have that ewe without a lamb.