Thanks for you thoughts everyone. As I said I have a bit of history with this customer and I should probably have gone with my first instinct, which not to even answer his call.
Summer before last I completely replumbed his 5 bed house; new boiler, unvented twin coil cylinder, solar panels, all new pipework and 25 rads. It was the first job of this size I'd taken on and asked him if he would consider doing it on day rate plus materials. He declined and said he wanted a price. I quoted £15,600 which he accepted. When the work was finished, he said he thought I was overcharging him and couldn't see the works being worth more than £13,000. He still owed me £8000 and after seeing a solicitor and thinking hard about taking him to court I settled on £14,000 because I didn't want to wait what might be a year for it to go through court.
Basically he's a player. He runs a Building services company and employs about 70 people. He's also an expert witness for the CIHPE. All the other tradesmen working at his house, which he gutted, rewired, replastered, replumbed and more... had hassels getting payed by him
After we settled, he said " this might sound strange, but I have a lot of time for you Mark, I'm delighted with the work you've done and if you ever want any help with wording your contracts I'd be happy to help"!!!!
Ah, it's a long story.. too long to write here. I didn't quote for the removal and refitting of the solar panels. I nearly joked with him "I guess it's pointless me quoting as you'll ignore it anyway".
We'll see what happens in court. It's more pride than anything. I suspect the court will set a figure somewhere between my price of £450 and what he has offered at £300.
Lesson learned... if your guts tell you to walk away, do it. If not get it down in writing.