Booked the Ferries!
Well, I've booked Poole-Cherbourg 23 Sep and have 4 days riding down through France (Day 1 90 miles, then 150-225 miles a day after that on the rural roads) and I'm planning to come back via Bilbao on 4th Oct. I thank Daffy for his discount code.
So if anyone wants to join me (3 places left in the cabin on way back), here are the details....