Grainger Games have just opened a new store (today) in the Merseyway Precinct in Stockport. It's not on their site at http://www.graingergames.co.uk
but the telephone number is 0161-477-1727 (as per my receipt).
Staff indicated that Mario Kart had only just been reduced to £25 that morning - so it could be the same in all their stores (as they've only just opened today). However, the price is £28.99 online (link provided for reference).
When submitting this deal HUKD highlighted a deals from almost 1 year ago, where it was listed at £24.99 in Grainger games.