Who did you make the cheque payable to?
That will tell you who the contract is with...
Once you establish that, then couple of options. Final letter than off to the county court to issue claim. If it is a company you can issue a winding up order. That tends to get things moving... If it is an individual than you can also either get a judgment against them or make them bankrupt.
However, in all these cases it is a matter of working out whether the person can actually pay you because you do have to pay fees to the court to get things moving.
But the main thing is who the cheque was made out to....