Why not go to the solicitor now? I mean, essentially you are a company- your company provided a service and was never paid for said service. Did you say that you have a contract with them?
Are you contracted to Creamfields the company or to the polish?

Actually, are the polish guys a seperate company? (subcontractors for Creamfields?)
In which case, who's name is on the contract, because it is them who need to pay you.
After that, the next step is to sue, sounds a little cold. However, I think not paying you for a job you did is cold.

Need any legal advice on contract law? PM me.