Enforcing Terms on a Contract

We run a media events company and we have a dispute with a client who signed our contract for advertising on the giant screens one of our events last year.
They then pulled out of the contract a week after signing. We informed them of our cancellation clause that states 60% of the contract value is payable by them. We then agreed to allow them to transfer the booking to another event. this year.
We want to make sure that if they fail follow through and sign up to go ahead with the event this year, we are able to enfirce the cancellation clause from the signed contract of last year.