I don't know if you have the same emissions equipment there, but I had this same thing happen when I was messing around with the charcoal cannister by the rear bumper...I had removed the valve from inside the frame rail, and it was coming in through the trunk (boot for you) of the car (I have no rear seats).
Check all the rubber hoses around your fuel tank.

 

argh - its gotta be the o2 housing -> down pipe.

still getting exhaust smell + soot on the front of the rocker cover

It looks like the donut gasket isn't making a good seal with the down pipe.

I guess I will need to take it to an exhaust shop to check out?

Cheers
D.

 

Is your new DP made to be used with the donut gasket? (Some are not.)

 

My car came with some "T-Pro's" DP (which the fitment SUCKS) and it uses 2 o-ring type flat gaskets, same metal gasket material you use at the cat.