I used a jet wash

[JPBoothy]

Thanks, but I prefer not to spray carcinogens around liberally, especially where the kids will be playing.

Please Always Read the Label
Don't use this product on impermeable surfaces such as concrete, tarmac, decking and paved areas, or any other surface, which is underlaid with concrete, cement or any other impermeable material

Make sure you put on very old clothes and if possible waterproofs too, as the amount of 'blow-back' from the cracks between the flags is unbelievable. Eye protection is a must!

 

[NorthernDave]

Go for the flame thrower/ gas burner opton.
Much more effective and oddly addictive...