After reading the topic about dodgy swimming pool works i think this will be some fair advice.

If your getting works done at your home ask the person quoting (or you have decided to go with) how much they are doing and how much, if any, they are going to sub-contract out.

We had a large pergola built and was told to pay for each person for the timber delivery, the painting, the building of the pergola and then a final payment to the company we chose.
Pergola looked great but when the builder gave me his final bill i gave him a defect list, tops of timber beams not painted, additional support / end brace required, trelliss broken

He knew i was/am a tradesman and just said 'Fair go i'll get them back' which he did within a week and i paid him the final part in cash.
this company is still working today and the pergola is now 10 years old.

With our pool we spoke to the installation guys who said they were that busy that were doing a pool a day but when we wanted to have a slate top on we were given a number of an Italian chap (Mario) who was unbelievable, old school tradesman . He said to wait a few months for the pool to settle then he'll do it, The pool people were going to add it to the cost of the pool and do it the following week.

Generally people think they can be builders by usings subbies and marking up, that's their version of project management.

Just a thought.