Provided you did not sign anything which created a direct contract with the sub contractor this is not your debt.
There are 2 ways for the sub contractor to be hired. The first and the way in your situation is that you have a contract with the builder. The builder then contracts the sub contractor to do work on the contract but for him to enable him to complete his contract with you. It is the builder that has the contract with the sub contractor not you.
The second way is that the builder puts together the client and sub contractor and they contract direct with each other for the work to be done. In this case the contract is between the client and SC and the client is responsible for paying the SC. this is NOT the position here. However because there are 2 ways of doing it this sometimes causes confusion
It seems to me that there has been some dispute between the builder and sub contractor about the amount the sub contractor should be paid by the builder. The sub contractor is sneakily trying to get you to pay the money which will leave you out of pocket.
Advise the debt collector this is not your debt and that they should check the position before harassing people, it is after all their job. If proceedings are commenced you will defend and counter claim for costs .
Advise them that should you hear from them again you will report them to the police for harassment and for attempting to obtain money from you by deception.
Any further problems come back to me.
I hope this helps. If there are any further points please reply I will be happy to respond