Legally, it'd probably be safest if you wrote to them to say that you are willing to move the sheep as requested but in order to do so, would need access for your trailer, and would they be able to unlock the gate for that.
It sounds as though there has been a falling out between you and the landowner. (No judgement either way, just going on their behaviour.) So you may feel that a letter, however politely worded, could inflame the situation further, in which case the snip off the old padlock, replace with new, and new key through their letterbox is probably the most practical solution. Almost certainly illegal though (whichever country you are in), so again, you may feel it could result in them taking matters further. If so then you may feel it safer to take legal advice before doing anything.
I didn't read it that you are unable to get to the sheep to check them, only that you need their locked gate opened for trailer access. If they are impeding your ability to check on the sheep daily then you may be able to get assistance from the RSPCA (or SSPCA as appropriate) to get access, on welfare grounds.