First thing is did he provide a 'quote' or an 'estimate'? A 'quote' must be stuck to, and an 'estimate' is not a fixed price. It's a rather underhand way of being able to charge you what they like if they provided an 'estimate' without explaining the implication. Check the paperwork, if the word estimate appears anywhere (even if alongside 'quote'), you won't be able to do anything. The window situation can be easily resolved if you offer to supply and provide your own. That being said you do sometimes need to take a step back and be objective: you may not think some work is necessary, but often some of the miscellaneous and unexpected work cannot be avoided. I would get another tradesman over to do a quote to finish the risk) I wouldn't mention a dispute at this point or else it will be very off putting) and see how the figures look. If they are quoting similar figures as your existing tradesman, unfortunately you will have to accept this is just one of those jobs where the budget isn't going to stretch to your requirement, and you'll have to lower your expectation.

Yes, ring up Gas Safe (hopefully he is registered). If he is a registered fitter with the boiler manufacturer you can put in a complaint about him with them too. If he is registered with a website like CheckaTrade you can speak to them too.The next thing might be your local Trading Standards and put in a claim as the Small Claims Court. Do not make any further cash payments, insist on an invoice and bank transfer. Only ever use a tradesperson who can provide a written quotes, invoice, and who wishes to be paid by bank transfer in instalments or after completion of the work.

You are not responsible to protect his profit from the job and him underpricing the job is his problem not yours. However you need to consider if another builder(perhaps even one with a higher quoted price) would ask you to pay for these things. What is the £11k for? Had you changed anything from the original plan? Did building regs require additional work not specified originally? Was extra work uncovered once they started the build? ( it is common to find unexpected rot or a collapsed drain and many other things that can’t be foreseen) did you add to the spec? Did any indecisiveness on your part delay his schedule? Did you start building much later than anticipated and materials had increased in price? Did you specify materials from another country that were affected by exchange rate fluctuations? If the answer is yes to any of these there is a reason why the builder is asking you for the extra money and a legal push back would (in my opinion) be ineffective. This £11k does also only represent a 5.5% overspend and any one who has been a self builder will tell you that is not bad.

Somewhere online they are available. I know this because I had a customer who printed one off for our company to sign. I promptly told her if anyone was signing a contract it should be her with a contract supplied by me. After all I am the contractor. I never signed it and presume she went elsewhere or never had the work carried out. Rather a cheeky thing to do IMO but I guess it depends on what exactly is in that contract.