I really feel for you mate, this job can be horrible when it goes pear shaped.
Get through this gloomy part however you can, if I were you I would consider writing it off completely, hopefully get the boiler back and break even on the next job.
I know you probably could take them to court etc. but do you want to go through that? The courts tend to be on the side of the customer from what I've heard and if you do take them and lose and then have to pay their expenses - how will you feel?
Move forward and learn, it may be an expensive lesson, but it will be a valuable one. You'll know next time to keep the communication up and to take deposits and/or have a cast iron agreement in place before starting.
I had a nightmare a good few years ago, spent a whole weekend replacing a bath that the customer had bought together with the taps they bought, all cheap rubbish, took the old bath out and the floor was rotten, I told them the floor needed re-doing and they asked if I could do it. I said I'm no joiner but I could replace some boards. They provided some crappy tongue and groove that was about 4mm shallower than the other boards and I begrudgingly fitted it with them (about 6 of them) watching my every move. Almost at end of job and the taps wouldn't seal, they were absolute rubbish quality, I'd had almost as much as I could take and when I told them the taps wouldn't seal and better ones were needed they started calling the flooring I'd done and blaming me for the cheap taps.
I picked up my tools and told them to **** off.
Anyway to cut a long story short I was 2 days labour down and about £100 in materials down but I was getting so low because of these arseholes that I had to leave before I did something they'd regret.
Took me a good few weeks to get over it and I didn't lose nearly half as much as you did. Maybe I could have taken them to a small claims for the materials and labour, maybe not. It didn't really matter I just didn't want to feel like that anymore, I didn't even want to talk about the job as thinking about it depressed me.
I have learned and I have moved on and it's just a bitter memory now which hopefully yours will be at some point in the future.