Have you discussed what you are not happy with & given him an opportunity to rectify the problem?
Is the amount you withheld proportional to what you are not happy with? Have you told him you are witholding some of his payment & what did he say?
I was in a similar position, gave him every opportunity to sort the problems (there were many) and eventually told him he was fired. We paid him (more than I would have liked). He did treaten to sue & we told him to go ahead, we'd be happy to discuss his shoddy work in open court - he happily took the cheque (oh, we made sure he gave us a full VAT reciept also so he couldn't just pocket the cash...agin, not happy but a mention of the tax man sorted that one out).
Yeah, I've been good about the whole thing. I had a pump replaced but eneded up without hot water. He said it was just a coincidence (obviously wasn't) so i got another guy in to take a look. Apparently a simple switch had been turned off. Despite plumber no.2 writing to plumber no.1 telling him this, I am getting all sorts of nasty letters demanding payments. (I paid €500, with €300 owed which I think is fair given all the agro (time off work, €90 for plumber no.2 fee and no hot water!)
I only had sinkful of hotwater for about a month. Plumber no.1 came back but couldn't find the problem and wouldn't accept any responsibility. Said it was just a coincidence (very unlikely!) and told me to get an electrician as the immersion heater was obviously broken.
As it turned out, the immersion was never broken and plumber no.2 simply switched the supply to the hot tank back on.