Ok, that sounds a bit better... but really any bought in stock straight onto Heptavac.
Lots of large scale farmers don't vaccinate their lambs if they plan to sell them all of before the winter, but I always do all of mine about 6/10 weeks after the last one was born, even if I sell them in October. It just seems a small additional cost compared to lamb prices of about 70 quid and more....
To get lambs into the Covexin 8 or Heptavac-P system, they have to have two shots, 4-6 weeks apart, neither being given before the lambs are three months old as the passive immunity from the vaccinated ewe would interfere with the new vaccination. Farmed commercial fat lambs should be away at 14-18 weeks old, so shouldn't really be needing vaccinations unless their mothers weren't vaccinated before lambing. Lambs being sold 'in the store' would normally be sold unvaccinated; the buyer can then put them into whichever system s/he favours. Breeding sheep may be vaccinated or not, depending on the vendor; if it was the other system to yours, in theory you have to give them two shots when it's booster time the first time to switch them to your system.
Often it isn't the cost of the med that stops a farmer giving a dose of whatever - it's the logistics of getting the sheep in for yet another handling plus the stress of the gather, move, treatment and journey back to field or hill. Hill farms tend to still work on specific gathers - crutching and sorting for tupping; tups off; pre-lambing / scanning; lambing (if lambing in-bye or indoors); clipping; weaning / culling / drafting. Here, when the sheep are in the pens for whatever reason, they get whatever is the next treatment due, or two or three if they're due or nearly and won't conflict with each other; BH hates having them in and not giving them something, as it means another gather and journey for them.
Having said all of which, once the older lambs are over 3 months old, groups of lambs are in reasonably regularly for being drawn for sale, so it would be easy enough to start vaccinating those remaining at a point. I think most of us find that the immunity from the mother seems to last into the back end of the year, so any keepers would be vaccinated along with the keeping ewe lambs at this point.