(this isn't an answer, more another question raised about it - sorry ! )
I have had vets say that the Heptavac can be given up to 3 months after the previous years dose, and is fine for the yearly booster - I have done this for my older ewes not in lamb, , so I can use the same pack for the younger pregnant ones. This saves buying more than 1 pack and wasting it. They were sure it was the same as dog vaccines, and up to 3 months over was fine, and no need to restart the course. Different from horses where 1 day over means you have to restart the whole course. Not sure why it is different - all money led, and don't even get me started on why human vaccines can last a lifetime with no boosters....
I have also had vets say that giving the booster a month (and not longer ) before lambing means a better transfer of immunity via mother/lamb way in blood before birth, and colostrum after.
Will be interesting to see replies. I don't object to spending money on stuff when they need it ( and to be fair, I'd sell the house if I needed to, for the dog ! ) but sometimes I wonder about the advice vets give, and the reasoning behind it.