As Anke expected, I wouldn't agree with keeping the vaccine for the second dose. The instructions say use within 8 hours as it's a live vaccine. At £20 abottle, why bother taking the risk and possibly injecting yoru sheep with something that doesn't work?
Once you've done the double dose, it's an annual booster after that, 4-6 weeks before lambing (most folk give lambs the initial double dose at about 4 and 8 weeks of age).
With small numbers, yes, there is an added cost but the vaccine is against clostridial diseases, which are soil borne so being a great stockman won't stop your sheep getting them.
I'd be looking to dose them for fluke at this time of year with triclabendazole (your vet might be able to give you a small amount) and I'd also be worm testing to see if they need wormed, being this year's lambs.