"Really Useful Box" branded 64L storage boxes were £11.25 each in store
at Staples Fareham, Hants. They had a sign saying "no further discounts" but as usual buying 3 or more works out cheaper - in this case £9 each. (3 for £30 with a further 10% discount).
The store had a "Store closing" banner above it but I thought that was soon to be the case with all Staples stores. Not sure if the same price in other Staples.
Online price is £12.98/£10.00 : http://www.staples.co.uk/64-litre-stacking-storage-box-clear/cbs/297998549.html
These are more expensive than many storage boxes, but they are much sturdier too and well reviewed.