£40 IS DIRT CHEAP.. I WAS GONNA HAVE MY MK2 DONE.. WAS LOOKING AT UPWARDS OF £500 (oops - exceuse caps!) to get mine done.
Apparently you have to be very careful when doing it because the heat from the sand/beads can distort panels, especially large ones like the roof.
Central Blast Cleaning do it, and i also spoke to another specialist company to do it who were more expensive. Both of them had done plenty of car shells and knew exactly when they were doing.
I chose against it due to the cost, but really wish i had now, after spending days under the car with a scraper and angle grinder to remove all the underseal and ziebart.