All lambs should get two jags, six weeks apart. They must be older than three weeks to avoidcompromising the passive immunity they get from their dams - assuming their dams have been injected prior to lambing. So if you haven't done them, do them now with two doses, 4-6 weeks apart.
If your adult sheep haven't been done either, you can do them at the same time - they need the double dose to get them on the programme.
Once they are ON the programme, they need an annual booster.
Do all sheep you have on your holding with a single dose 4-6 weeks before lambing, so probably February time, depending on when you lamb. This is the annual booster.
Next spring / summer, do your 2012 lambs with a double dose, to get them on the programme, then all sheep in February 2013 with a single dose (the booster).
So once you're on the programme - all adult sheep in February-ish with a single dose and all lambs with a double dose in summer. Simples.
Hope this helps but if it's not clear, do ask.