I'm not sure you have pressure washers in the UK, but, if you do, I'd recommend using one. I have an electric one that produces about 1500 psi at the nozzle. It can suck detergent into the water through a suction tube - no idea what the proportions are since I've never use the feature. I use the washer for cleaning the concrete patio, walkways and driveway at our house. Before I got mine, I used a neighbour's 3000 psi machine and it took about 1/8" off the top of the concrete!
Mine comes with a pressure-adjustable wand for more delicate surfaces.
Mine comes with a pressure-adjustable wand for more delicate surfaces.