Dragon fish sites (foxy bingo, costa bingo etc). Deposit £10 and get £30 bonus. Place as much of your £40 balance as possible (so start with games a few days in the future, and work backwards). As soon as your first game starts do not buy any more tickets. Winnings go to cash balance (a typical player would only play a few games at a time and end up buying more tickets with their cash balance before their bonus). Typical returns on bingo are 50% so buying £40 of tickets (don't put the lot on jackpots though as there'll be loads of players) should yield £20 of cash to withdraw, and a chance of more of course (I won £75 yesterday!)
Only do one a week though and withdraw before you open the next account - these are easy gubbings.