Its ok i sorted it
ill post up here just incase anyone searches for the same thing
Remove left side of the mechanism (plastic cover) looking at it from what you would expect to be the front
OK... The yellow plastic piece is a stopper which engages with the big black wheel to lock it... to free the black wheel, press the yellow piece down and pull at the belt until it is free. The system will only work when the unit is at 90^0 to the floor BTW once this is freed, youre done.
Very interestingly brilliant mechanism i thought