Agree with Chris B - pressure washer, fantastic for patios, but also brilliant on cars etc as well. Can get them for �49 odd, and well worth the money.
Yup - tried everything over the years and the only thing that really works is a pressure washer......but it does loosen the concrete between the slabs (if you have any) so be careful.
Oh, and it can get pretty messy, so old clothes and wellies are the order of the day!