A power washer and detergent, followed by a thorough rinse (using the power washer too). If you don't have any access to that recess, I'd create one...doesn't have to be huge. Leave it open with a fan aimed at it for 24 hrs before using PureAyre according to directions.
Fwiw, Lowe's has a handy dandy hose-end power washer with a rechargeable battery on sale for $99 right now. I bought one last year to use on jobs that make getting out my 'real' power washer more trouble than it's worth and I was amazed at how well it works.
--Peggie