There's also
this one from Zooplus. It's around £8 for 5kg (£4.99 + £2.99 shipping), or a little under £16 for 15kg (£12.99 + £2.99 shipping), which works out a lot cheaper than the one from Amazon (roughly 6.8ltrs = 1kg, so you're getting 34ltr/£8, or 102ltr/£16, compared to Supreme's 1.5ltr/£2.69.).
But, erm, never used it before. =D So no idea what the quality is like. (Anyone else tried it? If I needed sand soon, I would buy to try it out and report back, but I've still got enough sand to last a few weeks.)