Contractors invoice dispute

I recently employed a plumber to install an en-suite, we agreed and paid a final bill of £5k with another £500 to be paid on completion of snagging etc. He subsequently didn't return to complete this work but sent another bill for an extra £1k.bringing the total to £6.5k. I disputed this bill and now 10 days later he has sent a revised bill which has increased by another £2k with the threat of court action if unpaid. How do I resolve this?