I have given our patio slabs a clean with a pressure washer but it's left some black marks,can anyone recommend a patio cleaner which will help to clear them
Sodium hypochlorite from a farmers' supplies or similar is the best. Dilute 1:3 with water, sprinkle on, wait about 10 minutes and then hose off. Be careful, as it has a strong bleaching quality - farmers use it to clean milking parlours.
if concrete and NaOCL does not work (it is very corrosive and chlorine fumes can cause internal burns) use a standard patio cleaner - most normally have a dilute acid in them - normally HCL (Hydrochloric acid)