Withold payment until the job is done to a satisfactory standard by the original plumber and dictate that you want it done within 28 days (often considered a reasonable time in law), otherwise tell him that you will get a professional plumber to put it right after that time and will sue him for any additional costs over and above the £1500 pounds that he says you owe him (including time off work, phone calls and letters etc).
Also tell him that you will be contacting the Trading Standards about him if he doen't put it right.
If none of this works, I would also recommend that you put a nice video on Youtube (after any possible court action) with a heading like "Plumbing Disaster" and name and shame him.
If none of this works tell him he can take you to court to sue you for the money. I think once a court sees the shoddy work he has done you will have a very good case for not paying him in law.