It depends on the bank really, some banks will charge up to £10 to cancel a cheque, for example, if you have merely changed your mind about a product you purchased. However, if it has been lost in the post and you wish to re-send the cheque then they will probably not charge you. I advise you ring your bank in the morning though and find out what their policy is.
EDIT: Also, cheques can be cashed by the recipient up to 6 months after they have been issued, so during this time you should no longer consider it as your money, if you cancel the cheques while the recipient has not yet cashed them you can be sued for fraud.