hi there,
,many years ago i used to be an area manager for a petrol company, we used a company that would come round every six months, and jet wash the forecourt, the gulleys, etc and then empty the interceptors.
some of the pitfalls,are that on brick forecourts, they would sometimes blast the sand out from between the bricks, on concrete foreecourts, the pressure washing would just lift the surface grime, it wouldsnt take away any of the soaked, or ground in contaminates.
the other think you have to be careful of is not to flood the tank top wells, admittedly nowadays everything is on 'offset fills' but depending upon the age of the station, the amount of water ingress to the tank tops, can be a problem.
it all depends what the cleint is looking for, if theey just want a spruce up, or if they are looking for a forecourt back to when the site opened its doors on day one.
good luck
regards
martin