Oh dear, that's annoying.
In theory, if they were with a tup in August they could lamb any time from January - it's 147 days, usually calculated as '5 months forward and 4 days back'.
In practise, in Cumbria, Blackies probably wouldn't get busy before October, which would give March lambs.
I don't know where you are and whether the sheep get active earlier thereabouts.