Why does your zafira keep stalling when the revs go to idle speed?

If it's a 1.8 with a paper element oil filter....

There is a small hole in the throttle body that get blocked up with carbon deposits. This can cause stalling problems.

There is a small pipe going to the side of the throttle body (about 10 mm in Diameter) remove this pipe and use a carburettor cleaner to unblock the hole.

There is a factory re-work to address this problem, but it's too complicated to spell out here.

A less likely cause it that the throttle body is it self faulty, and needs replacing.