Do you have any agricultural merchants near you? If so have a look at the sort of ones they sell.Compared to halfords/DiY shop pressure washers you will find you get a much more powerfull and robust washer for your money-but it just wont look as pretty.
I found this guide too:
The following examples list common tasks for pressure washers and the pressure needed to do them:
Car wash: 700 to 1,000 PSI
Cleaning decks / house sidings: 1,000 to 1,500 PSI
Heavy duty equipment and agricultural uses: 2,000 to 3,000 PSI
Concrete surfaces and heavy industrial uses: 3,000 + PSI