If you're only levelling the floor then ripped plywood works well (ideally inch, 20mm would do). Not sure whether it's a cheap option with current prices for ply ![Frown :( :(](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
EDIT 4" strips screwed would do, you'd get 29 metres out of 1 sheet of ply![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
EDIT 4" strips screwed would do, you'd get 29 metres out of 1 sheet of ply