Just checking Dave that you have done the obvious which is to undo the bolt and knock it inwards to release the clamping effect.
On some fittings there is a "Rawlbolt" type of fixing which splays out to grip the inside of the bars when the bolt is tightened. You need to reverse the cation to loosen it.
Thanks Brian, yes I gave them a tap inwards, but they wouldn't budge at first. I have now removed one, just need to persuade the other one to follow suit!