Here are a couple references for others who need help with the dreaded p0491 & p0492:

My troubleshooting and repair of codes P0491 & P0492. - Bimmerfest - BMW Forums

https://www.bavauto.com/newsletter/2...newsletter.pdf

So, after some needed advice from Weasel and reading the two articles (above) I realized I have two problems. The secondary air check valve is faulty and there is a crack in the tubing from the electric vacuum control switch/solenoid to something behind and under the manifold in the rear back of the engine bay (possibly the reservoir). The tube has a non return valve connected to it. # 7 in the picture below.

http://www.realoem.com/bmw/diagrams/v/u/73.png

Does anyone have any advice which would help me get to this tube in the back of the engine bay? or any other advice would be appreciated.

Bumping this thread and adding an updated link for the valve
RealOEM.com - Online BMW Parts Catalog

Looks like BMA sells a PIERBURG valve - will look again through the thread but has anyone any experience with this brand? Is this the BMW OEM?