We do have a contract with them, but it's hundreds of pages long and I wouldn't be able to scan and email it all to you.
I have checked through the payment and pay-less sections of our order and cannot find any clause to say that they are able to withhold money from one job to pay for another.
Please see the attached, which is all that I can find in the contract with regards ***** ***** and deductions. It all seems to be contract specific, which leads me to believe that they can't withhold monies from a separate contract.
If, however, it transpires that this is common practice with other principal contractors - maybe they can???