watched a home improvement programme recently with that big builder who worked with Alan Titchmarsh on the garden programme...
Anyway, he was advising on how to clean patios and he had a 5 litre container of caustic based paving cleaner which he decanted into a watering can and sprayed onto the paving, leaving for 10 mins or so , then he hosed off...not jetwashed...it was amazing, they came up like new. Apparently, you can get this stuff cheap at any builders merchants/B+Q etc.
Steve