Unfortunately with all jobs there is the nitty gritty of contracts and the agreements with payment times and amounts to be made. They are not much fun, but if they are not agreed to properly they can cause you a major headache later on – they're worth getting right from the start.
The bigger the job, the more important it is to have a written contract. It comes down to risk in the end. Ask yourself “what do I risk if I do not have a written agreement as to exactly what will be done for the money I pay?” If you are having a lock repaired most people will call a company and have a verbal agreement on the cost. Consider an a appropriate document depending on size/cost of the project. The bigger the cost/project, the more formal the contract - something you will need to consider if you want to use a solicitor.
Nowadays there are many templates online that you can use and are easily searchable by typing something like 'Building Project Templates' or 'Trade Project Templates' into your search engine.
Never pay for a job before it is completed and never pay out more than the value of the work done to date. If you agree at what points/milestones a payment is due, you will be able to better control the project and see if there are any differences in the bill versus the estimate. You can then discuss them based on the estimate and avoid major shocks when the final bill arrives. Discuss discrepancies early and ensure that you are not surprised. If the tradesperson / company leaves it too long before explaining cost differences, request more frequent updates on spend.
To ensure all work is completed, never pay the full amount before you are satisfied the work is complete. It is good practice to define up front how much the final payment will be and what criteria will be used to define when it must be paid.
Please remember that cash flow is an issue for many companies and if yours is a long job, they may have to pay for your materials bought for your job, so when it is due please pay promptly.
Retain a final payment of the overall cost of the work until after the entire job has been completed to your satisfaction – then pay it promptly.