Going back to the original question, it is not possible from such a brief description to make any sort of diagnosis. We can all guess at what it might be but without a far more detailed description no one here can say for certain - it could be any of several things.
It is very unlikely that the problem is poisoning by your neighbour - there are many far more likely options to work through first. It is probable that thinking about the poisoning has stopped you from seeing the real problem.
Your best bet Worzel has to be to take your sheep down to a large animal vet practice for advice. If the sheep are dying that quickly then you may have lost the third one already. The rest of your flock will follow. We have found that our vets have a very reasonable rate for farm animals, especially if they are taken to the surgery. Your animals may well be dying of something which could be treated if you dealt with it promptly. A PM would show up what it is, but by the time it's done the rest could be dead too.
I don't want to add to the feeling you must have by now that everyone is trying to push you to do something you can't afford, but I really do think that you have to bite the bullet now and get your sheep to a vet. Keeping sheep organically cannot be an excuse for neglecting them - they still need treatment if they are ill.
There is nothing like the PDSA for farm animals - the SSPCA or RSPCA would treat your animals but they would also take them away and you would not be allowed to keep animals again.
When you decide to keep animals, you have to take on the responsibility for their care. It's great when they are healthy, but once things start to go wrong you have to pay for help. It's an inescapable part of animal keeping. As someone else on here has pointed out, very few of us keep animals on a large scale but we all accept that when they need professional treatment, we have to pay for it. You might be surprised at how little it will cost if you take the animal to the vet rather than calling her or him out.