Condition is going to be a big factor here. If its been ridden a couple of times a year, and is immaculate, it’s going to be worth twice as much as if it has been ridden into the ground and everything is worn out.
FWIW, I think £650 is a bit optimistic. Usual rule is 50% off the value in the first year, then 10% off every year after I think. Though this isn’t always hard and fast, and the Fuel did have a good following (I had one, for a short travel XC bike it was the nuts at the time!).
It’s probably worth more broken for parts, especially with a full XT groupset. The biggest limiting factor in terms of desirability for any future purchaser is the Manitou Skareb forks. Nobody in their right mind is going to want to touch those these days.
With everything I’ve said above, I’d expect it to fetch perhaps £400-450 if it was sold on ebay, perhaps £500 or more if it was broken down for parts and sold individually.
Also, what size is it? If it’s a Small, or an XL, it won’t fetch as much money as a Medium will…