You will be pushing on to do that James !!
Anyhow, been looking at it a little more!
Its looking for me, best route is going to be Plymouth - Roscoff for £ 319, (2 persons,& cheapest cabin/bunk) overnight, getting in at 10.00 hours then 540 odd miles, in one hit, so down there around 22.00
I am working on 26mpg so total cost there and back of around £520
Going via Dover is near the 1000 miles as compared to just over the 600, so 800 mile round trip saving, (£150.00 diesel),
Chris, I know you use the funny fuel,, but to do 1800 miles on £ 160.s worth, ain't bad going !!