Buy a couple of rubbers (erasers) and use a contact adhesive to stick them to the underside of the seat. WH Smiths or Staples etc sell them in white or various shades....
Re B&Q. Goods are supposed to be "fit for purpose" and Jo Public wouldn`t expect to have such an issue so soon after purchase. Ask your local Trading Standards what they think (in Tel directory under County Council or Local Authorities banner).
A work colleague told me someone on his team had no satisfaction from B&Q and so started to dismantle the requisite part from a bathroom display. Bit drastic but it worked as they quickly supplied a new part!
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