530 drinking too much petrol - any suggestions?

[blackbeamer]

Some help appreciated here guys. My E39 '03 530 is drinking way too much petrol. Normally I get around 22mpg but past weeking averaging 15mpg. Recently had a service II and had a problem with the ignition that was sorted out. Could either of these have contributed? Looked for leaks and couldn't find any evidence of it. Any and all thoughts welcome before I take it back to the dealer!!