Simon,
If your grips of of a stretchy rubber material then you can break the seal with the handlebars by pushing something long and narrow in to make a gap into which you squirt water/washing up liquid and then rotate the grips.
You appear to have ergon grips from the photos. These have an allen bolt clamping them tight at the outward end (see photo). On mine I simply loosen the bolt, rotate the grips and tighten the bolt.