Just checked on Booking.com and it's a pretty grim scene- very little in the £200-£300 category, and they look like pretty dreary places, but they are there. Most places are in the £500-£700 category, which is pretty extortionate, even for London. The Holiday Inn Express in Limehouse is about £600!!!!
My advice to anyone who has the misfortune of being caught out tonight is to book a hotel in Reading. You can get a coach there quite easily and there is plenty of availability sub £200. It's not ideal, but it saves you having to pay silly money for frankly mostly bang average (at best) hotels.