Employing a good, reputable contractor in terms of price, reliability and workmanship will greatly reduce the potential for problems, and should there be any, it will be far easier to get these rectified.
Please be aware that when carrying out refurbishment work to 50% or more of a roof’s area that either a Competent Roofer (a Competent Person Scheme for roofing) must be employed or your Local Authority Building Control must be contracted prior to work starting. This is in order to confirm that the roof should be upgraded to meet thermal requirements of Part L of the current Building Regulation.
Never, ever agree upfront to pay in cash.
Letting people know you have cash lying around your property leaves you vulnerable to unscrupulous individuals.