England poor service: do I have to let the trader redo the work first?

Citizens Advice says services must be carried out with reasonable care and skill. If the service was poor, you normally first give the trader the chance to do it again if that is possible. You may be able to go straight to a price reduction if repeat performance is impossible, would take too long or would cause significant inconvenience. Official source: Citizens Advice. Guide: https://getinstantlaw.com/poor-uncompleted-services