Citizens' Advice would tell you their take on things and probably advise you on how best to proceed. However, given your description of previous attempts to get things put right it is (as you indicate) time to be firm.
In your place I would obtain the name and address of the local manager plus that of the central office and relevant manager, even CEO. Write a letter to the former, copy it to the latter, and list everything that you are dissatisfied with (leave nothing out unless you don't want to pursue it) and include descriptions of all the times you have raised each matter with them (I hope you have a good record of the exchanges).
Tell the manager that unless you receive Wickes? full attention and prompt correction, you will start legal proceedings. Be prepared to take out a small claim in your local court - you can bring this yourself without a solicitor, but make sure you describe your claim thoroughly and include everything, also compensation for your troubles. Normally there will be people at the court who will guide you through the process.