If you are staying at Lamlash there is an argument that says (Assuming yo uare not holidaying solo) speak to the rest of your team and meet them at Lamlash pier - the cost of taking a rib over on the ferry (you have just increased your deck usage by about 110%) why not thow it in at Ardrossan or Largs marina and motor it across while your other party members take the car? Unless yolur O/B is particularly thirsty it is (certainly was) cheaper to burn the fuel.
If you are staying at Lamlash you would be 100 times quicker motoring round to Lochranza - it's an hour top to bottom of the island by car, regardless whether you go via east or west roads. Then add trailer......
Grocer is certainly your man for the area!