We've had ours done twice within weeks - the beggars got away with almost 1000 litres each time... We were rather unhappy bunnies.
We now only buy 500 litres at a time - so not such a big loss.
A froiend was changing out his red tank and offered us the dregs of 'dirty oil' - took it to run through the heating, and lo and behold by the time we got it home on the Sunday and went to load it into the tank on the Monday somebody had been in at it. would love to have seen their faces when their vehicle broke down. kept everything crossed that it damaged the engines.