Hi, Simon. Tech Tip: the seat adjust rails are mounted on different thickness aluminum stand-offs at each end to slightly tilt the seats, and to keep one from tightening down the rails directly on the carpeting. There are small square holes for each spacer. If you remove them, the carpet compresses into the holes in the carpeting below the bolts and causes the thin rails to bend. Then the seats become hard to adjust, even with new rollers.
Some misguided souls think they can lower their seats an inch by removing the spacers, and they find the seats practically won't move afterwards. If you must lower the seats a little, cut the carpeting open the full length of the rails, then you can bolt the rails onto the bare floor without trouble. good luck-