This is the link address to the data sheet
http://www.vmd.defra.gov.uk/productinformationdatabase/SPC_Documents/SPC_108324.DOC .
Basically you can't give it less than 3 weeks. I assume you know the smallest bottle size is 25 doses and it is ineffective 10 hours after opening the bottle - which makes it very expensive if you only use part bottles.
I had a lamb die after vaccinating this year - apparently this is not "uncommon" and not just because the lamb was under the weather and not spotted ie it really was completely healthy. After posting about it on another forum, I am seriously thinking of moving away from using it next year and using a vaccine against pulpy kidney which is much cheaper and then monitoring the lambs even more closely for pneumonia and treating accordingly. It's on the talk to the vet list. I prefer vaccination to antibiotics but..........
Your ewe presumably still has some cover for herself but I don't know hoe much would pass into her colostrum, again you would assume not much or why do we do a booster every year? A ewe that misses a year has to have the double dose again.
I would ask the vet, I've always found mine quick on the uptake when I start talking about the costs involved for a small flock. They won't do part bottles of heptavac but will sell small quantities of lots of other things.