Building Society cheques are not the same as Bank Drafts. Don't let the car go until the funds have cleared. Tell him this before he turns up to save a wasted journey. If he's genuine he'll wait a few days till it clears or he'll arrange a Bank Draft. Don't feel pressurised into letting the car go before you've got cleared funds.
As someone already said..........he has to go into the branch to get the 'cheque' so offer him �50 off for cash to be safe. If he doesn't want to safe himself �50 then don't let the car out of your sight till it is 'available' in your account. Don't just accept your current balance if using phone banking as it can still bounce. Wait until you know the money is available.