Ah, so you know the ground and weather reasonably* well. Do you know what size panniers the bikes come with? Might seem like a daft question, but if you do fill a 120L bergan and have 60L of pannier space ... Can you pick up supplies as you go or do you need to carry everything from day one? (Genuine question, I don't know Arran at all but if you can organise fresh supplies it'll ease your burden) Another daft question ... Do you usually use a cycle?
* Within which the unspecific bounds with which the four seasons are very loosely bound .
we will have 30L of pannier space once we get there, and resupply is fairly intermittant but ive worked out that at our longest strech we will need to carry 3 days worth of supplys but i know fresh water is avilable throught the whole trip. I cycle everyday 12miles to and from work, but this is the first trip of this nature i have done