Claiming unpaid wages - how?

[JanePurdy]

My DP did a shift with a locum agency at the end of November. He still hasn't been paid for it. Every time he chases it they promise it will be sorted this week. They have paid him for other shifts since then. Would the only option be to take them to the small claims court (not the preferred route as he still gets shifts from them!) or is there some other process?

Thanks.