Noz makes a very valid point. No matter how upset and frustrated you are by the poor standards and we all understand your reluctance to have the original contractors back, unfortunately if you get a third party in to investigate the work, the original contractor can deny their findings and claim that your new contractor has messed up their original work. Within the eyes of the law it is perfectly reasonable that a trader may have made a mistake on some part of their work and if they have, it is also reasonable that they should be given opportunity to put right any discrepancies identified. But it is unreasonable to expect some compensation from the original trader when you have already employed someone else to fix something they have not had a chance to fix themselves. Consider if a customer complained about some of your work but took the car to another mechanic first to have it fixed, then came with that bill to you to pay for them. You would quite rightly decline to pay saying you were not given any opportunity to fix it yourself at your own cost, which may also be cheaper than the new bill. I would think if you are using another electrician, you need a third party observer from the builders there while you have your own electrician investigating the installation, or it could end up one big denial or responsibilities argument.
Doc H.