"Title of the goods does not pass until paid for in full" these are the simplest express contractual terms you must use in order to lawfully justify the reverse engineering of work and materials you have previously installed but not been paid for.
Sounds easy? Not so! As the OP was acting as sub contractor for an agent his contract was with the agent not the end user. To reverse engineer the work carried out he would need to get the permission of the end user to enter the premises and carry out the work. He cannot just storm in and do it. A quick call to the police would result in him being removed by said force at the very least, charged with criminal damage and or breaking and entering and crim damage at the very worst.
What he could do is find out if the business has paid any money to the contractor and if so how much and if not why?
Also, a quick check with companies house should establish if the business is Ltd, still trading, names of directors, any CCJs awarded etc.
Cases like this are very frustrating for sole traders and I would advise you all to brush up on the changes to the latest changes to the Consumer Rights Act October 2015. If you are to have any chance in legally claiming monies from customers you must follow the correct procedures for estimates contracts explanations of cancellation rights, terms and conditions, etc. Failure to miss out on what is now the customers rights will result in a claim being denied. Get the paperwork right and your chances are much better.
More info
http://www.which.co.uk/consumer-rights/regulation/consumer-rights-act
In particular pay attention to the section on what is now considered in law to be "service" and what is expected of the contractor.
One important change is the consumers rights against the retailer for the first 6 months of ownership. It's worth mentioning that it is the business that invoices the customer last who is the "retailer" not automatically the wholesaler. So if you buy a shower from Plumbase and then invoice the customer for the shower, you become the retailer. For this reason many operators are now asking the customer to deal directly with the wholesaler or retail outlet.