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You can smell rich running very easily. Another thing to check is that your MAF is working correctly and there are no vacuum leaks

Nah, i always run it on a half tank. More concentrated

Thanks. I'll remove and inspect the MAF and check over the pipework again.

CO is normally a sign of air entering the engine after the MAF, as its already metered it so it fuels appropriately. A split hose somewhere etc can cause all sorts of issues

Are there any particular failure points in terms of hoses to check?

Might be totally unrelated but when I cleaned my throttlebody it cause my car to have errors P0106 / p0170 / p0171.

When I did the throttle body alignment cured everything

I have no error codes at all.

I ran up the engine to full temp and gave it some high rpm running today.

Old NGK BKUR6ET plugs were replaced with ZFR5P-G.

It's not running rich:

Failed again on emissions after a new O2 sensor and a bottle of Cataclean. No error codes. Cannot find any leaks but is it worth replacing the brake servo hose?

Not if its obviously not leaking. Is the vacuum valve on it working correctly?

I removed the hose to check visually for any splits, etc. but could not find any. The openings indicated by green ink pass air one way (by lung power at least). There is no air passing either way through the opening indicated by red ink (which connects to the air box).
The connector to the brake servo did seem a little loose but I guess it's sealing OK nevertheless.

I don't have a high pressure line to test properly. Might try and rig up a bicycle pump and use a bowl of water.

Any other possibilities for the emissions problem? Inlet manifold?

Possibly. Its air coming in after what the engine is expecting via the intake.Smoke test might be helpful

Approx how long would it take to remove and inspect the inlet manifold?

hour tops, its reasonably easy to remove on these. Whilst you are at it, check over every visible pipe, and also make sure the exhaust manifold, cat, and flexi arent cracked

Removed and checked the breather hose from crankcase to inlet manifold. Found that the crankcase end was not seated properly. Fitted a fresh o-ring and refitted. Not sure how this came adrift, perhaps the plastic ring was not positioned correctly.

Interesting. When is the retest?