you can get a cleaning solution from B&Q dilute it into a watering can, sprinkle whole area with solution then wash in with a stiff broom and rinse with clean water, i find this is great for decking too
A few questions. What is it made of (Wood cement brick stone). Is it slippery when dry as well as wet? Is it slippery when clean as well as dirty? Has it ever been painted,stained? Different procedures for each of the above.