Cheers Boohugh and Fisherking, that makes sense.
I thought it might be the case, but wanted to make sure I wasn't missing a way of expanding.

The ships do show a coloured surround, but it's the paler green which I think denotes a 'zone of control'. The brighter movement zone doesn't seem to exist. Now I just need to decide whether to leave the ships there (as they occasionally get to intercept fleets using the Channel) or if it's not cost effective.