Before everyone gets carried away with this, please bear in mind that landowners might not appreciate piles of soggy muck left near the entrance to the site. Equally of issue, would be SEPA's (inevitably dim) view of operating a car wash in a field, oily water run off, pollution risk and such like. I only mention this as I am the unfortunate soul that has had the pleasure of dealing with SEPA and other similarly unsympathetic bodies over matters like this in the past in relation to our club activities and believe me, we really don't want to get their attention again!
Alan