Paul

Firstly don't worry about the cheap cat at this point, These readings are to far out to be a problem with the Cat alone. The lambda reading should pass/meet MOT limits without a cat, so get that right first.

I would say that 0.6 v is low for engine that is that rich. what does live data show ? is it a good quality sensor fitted.

What is live data showing on MAF, CTS, Lambda etc ?? Any fuel trim info ?

Other points to eliminate are fuel pressure and pre cat air leaks.

Basic checks, compressions, manifold vacuum, inlet leaks, plugs, oil quality, air filter etc should be carried out.

Cheers Aidan