Hi Chris,
I have just done mine for the first time so its all new to me as well. But this is what I have read on line.
Heptavac P Plus sheep and lamb vaccine for the active immunisation of sheep as an aid in the control of lamb dysentery, pulpy kidney, struck, tetanus, braxy, blackleg, black disease and clostridial metritis caused by the above listed organisms. An opaque fluid vaccine combined 7 in 1 clostridial plus pasteurella vaccine
My understanding is that you give it 4-6 weeks before the ewes are due to lamb and that in turn is passed on through the ewe to the lambs. The lambs then need to be done themselves. I think thats about 3 weeks after birth although still need to read up to confirm. They need a second dose which I think is about 6 weeks later. After that they just need one booster a year so my one ewe that isn't expecting got a jab at the same time as my mums to be.
Don't take my word as gosple though as I am still learning. Someone with more experience will confirm or deny I am sure.
Sally