Sorry to read this. What area are you? My concern would be that they might be back for more.
If the field is close enough to the house (or an outbuilding with electricity) there are some good long range cctv cameras available, but if not close enough, maybe take a look at investing in a remote field camera or two. Buy one with an invisible I.R., hidden in the hedgerow they are difficult to spot, throw off no light at night (or day) and as are so difficult to see are unlikely to be stolen. They can be set to take photos, video, or both. You can also buy ones that connect to the Internet so as soon as they are triggered they take a photo and send it to the mobile/s of your choosing. With one of those cameras if someone did decide to pinch anything (sheep or camera) you'd have a photo of them doing it. Another advantage with that model of camera is you can alert the authorities whilst they are still on site.
Hope all goes well.