What you call cadding we call awnings.
I think you call them UVPC sidings.
But
philsimm named them wrong.
I've just done an awning today. I did a post on this some time back. You can stay dry if you work correctly, except when you are doing the insides or underneath. Take a spare t-shirt.
The job difficulty depends if the covering is vinyl or fabric/acrylic mix. Do not use bleach whatever you do. If you do not have the right chemicals do not attempt it unless you want to fork out for a new awning.
You could cause a considerable amount of damage from promoting plastifier migration to just pushing the dirt further into the a woven awning.
Is it colour safe? i.e. will the colours run when you wash it?
Have you done spot tests in inconspicuous areas to check the chemical you are using?
Do not use a pressure washer unless you can take the pressure back to under 60 psi otherwise you will stretch the awning. I use my waterfed pole.
Never wet the awning before applying the chemical.
Always work from the top down.
Never attempt to clean too much awning at once, a square metre at a time will suffice. Failing to do this will sink the chemicals into the awning and will set quite soild. At the same time you will have to let the chemicals do their work.
Work safely, these chemicals are slippery.
If the awning is ripped, get a waiver signed, you will only rip it more (this includes cigarette holes).
Finally you should expect to get 10 quid a square metre+vat.