Clematis will always tend to shoot from its last growth, leaving a bare base. So plant a low-growing shrub at its base to cover its legs, or allow your clematis to ramble over the top of something very vigorous.
I've used a watered down bleach based solution and yard brush and that did the job a lot quicker than the power washer.
The chlorine will render the surface nasty to the green regrowth so will stay cleaner for longer. Jeyes fluid works too.
I would advise you to trace the builder responsible. Many firms now have Trustmark or CPA (consumer protection association) membership and that can cover their workmanship in an insurance backed guarantee for 10 years. Worth checking if you're now facing a major problem.
In the meantime, the...
I ordered a garden office, as somewhere I could peaceably work - away from the distractions of my house. The Bedfordshire company I ordered from assured me that I would NOT need permission so I didn't seek it. I had a visit from the planning officer last week who insists I apply for...