Excellent price for this game, checked online from my phone the barcode scanner price checker thing and cheapest was £25, Seems to be selling on ebay for £20ish and blockbuster were paying £10 for trading in so you could buy play and then trade in.
Not many Alworth stores around and unsure if this was a price error. Xbox 360 version was priced from £40 down to £20 so think it could be an error this being £20 down to £10 but it did go through the till at this price when scanned so possible that the xbox version didn't get reduced.
Here's a store locatorhttp://www.alworths.com/storelocator.html
Worth noting they also had Little Big Planet Game of the Year Edition £10, Kane and Lynch 2 limited edition £10 and Alpha Protocol £10 but didn't think these games were worth individual deals.