I've had a rough idle for sometime now but today I found a inch long crack round two of the pipes on my exhaust manifold so could be the cause of my idle problem? I know the MAF has gone now and i've got a replacement on order but was wondering if the manifold was the underlying problem i've been experiencing?

 

The MAF sensor is the more likely culprit. With a leak in the exhaust manifold the car can still idle but you will lose spool and boost. Generally the performance will be lacking.