It's a case of live and learn, how much do you want to keep the contract? £65 is hardly a massive contract that you cannot afford to lose, must be probably 4-6 hours work max.
You've c0cked up on your pricing, tell them so and tell them why you want more money off them. If they are not willing to work with you give them a months notice, learn by your mistake and move on.
I would suspect that this is a verbal contract and there is no termination date, a months notice should be suffice. The only thing that will suffer could be your reputation.