There is no contract between you and the roofer. The roofer was in effect an employee of the builder (subcontractor). In exactly the same way that you would have no recourse against the employees personally you have no recourse against the roofing contractor.
If you threaten court proceedings the roofer may come round to do the job rather than has the trouble of going to court although ultimately you are likely to lose unless the judge can find some kind of contractual arrangement between you and the roofer.
If you do want to go as far as threatening legal proceedings and it still doesn’t work, he may do the work once you issue them.
You can issue legal proceedings online by following this link.
The court fee is £25 which you will lose if you are not successful but will recover if you are successful. There is a further fee of £25 if he decides to defend it and it goes to a hearing. You will also get that fee back if you are successful.
In view of the risk involved and the potential court costs, for the sake of £100 you might want to bite the bullet.
I’m sorry this answer is unfavourable for you.
Can I clarify anything?