hi john
we do this kind of work , we use a hot pressure washer the one you saw being used at freeport we use various chemicals as you will find some work really well on some surfaces but not at all on others , we have found the most hard surface to remove graffiti from is natural stone and sometimes end up putting soda chrystals through a sandblast attachment to remove the stuff but you have to be carefiull as the stonework can be easily pitted.
there is a company that do a training course for this work that i was looking into, it is a three day course and i think it cost about £500 but at the end you you are fully certified to work for councils and national trusts etc, when i am next at the office i will get their name and email it to you .