hi have you checked the rubber piping to the vacumm pump, you can get at it from the small inspection cover in the near side front wheel arch. mine had perished at the connection cut it back pushed back on all ok . if you cant see any thing there you can remove the near side front light cluster only 3 torx screws that gives you access to remove and examine the whole pump this is held on with 3 smaal bolts its a bit fiddly but not to hard. before you try all this dont wish to appear rude buthow have you tried to activate the cruise control have you pressed the button on the end of the stalk a few times in quick concession i have to pump mine about 4/5 times to activate