im cleaning the breather unit that sits on top of the plenum which is straitforward but should i clean the pipes attached to it & can i clean them in situ or do i have to take them out?

Yep - those pipes and the box that they are attached to at the far end.  I found access pretty awkward. 

If you spray cleaner into the pipes while they are still attached, it (and the gunge) will end up in your engine oil, which is a bad idea even if you do change it afterwards. 

Full details in the maintenance guide here:

http://www.omegaowners.com/forum/YaBB.pl?num=1285583967

I might add that when you are spraying carb cleaner into the breather bridge, the straw on the can tends to fall off into the bridge.  And it's a bu99er to get it out again.  So hang on to it tightly or don't use the straw at all.  Don't ask me how I know this.    :)