Yeah, well Jane summed it up really. They don't look very weathered. They look just about the right shape and size for fence posts. There are loads of stone fence posts on Orkney, so I was enquiring after something a bit more solid (metaphorically speaking).
Basically we should start any enquiry by say, "why do we think this is prehistoric?". Otherwise things would be barmy. We can't assume stuff is prehistoric just because it might be.
K x