Well, at that size and age, they shouldn't have any trouble lambing Manx X lambs, so if they are 'up the duff', it won't do them any harm.
I've no idea if the vet can inject/abort sheep as they can cattle, but if you didn't want them to have crossbred lambs at this point, that might be an option.
For us, it's too early for the ewes to come a-tupping, but perhaps it's not too early where you are?