I guess it will just be a case of see how things are when the time comes to renew. My fiancee renewed last night and we agreed to multicar, which reduced her premium by £120.
The girl at Admiral, who was very pleasant, explained that when the time comes to renew my policy (August) if we don't agree that the multicar policy is value for money then we can opt out at that point and my fiancees policy will increase by the £120. So I will just have to wait and see what happens when the time comes.