I'd go with the 'Plumbing Repair & Maintenance' category, making it clear in the job description that it's a complete re-plumbing job that you're after.
I hope this helps!
The Ask an Expert Team
Projects that add value to your home - Phil Spencer
Looking to extend your home but don't know where to start? Property expert and Rated People ambassador, Phil Spencer, shares some thoughts on why loft conversions are such a popular way to create more living space and add value to your home.