I think I read that Mr Foakes was on his way to WHF from his cottage close by,,at 4am,in readiness to work,until he saw the police cars around and realised that something was wrong. He very likely spoke to one of the officers that morning.
So yes,,they all began early because of it being harvest-time. There'd have been no need for Sheila to have been up,as she wasn't working and was purely on a break with the boys.
I bet Sheila wasn't relishing the idea of the boys going away either, with Colin and the girlfriend. Poor woman had got it into her head that there might have been some reconciliation between her and Colin.