I think this isn't good;
Since the car has a decat, I thought maybe the Lambda sensors were faulty and need changing as I got 2 codes when checked with a diagnostic tool, these are: (P0032 - OXYGEN SENSOR HEATER CONTROL CIRCUIT HIGH (BANK 1 SENSOR 1)
(P0037 - OXYGEN SENSOR HEATER CONTROL CIRCUIT LOW (BANK 1 SENSOR 2)
Now I should be running with a Lambda, though mine is 1 before the decat/after. I'm not aware if there is one in the manifold, but doesn't seem to be as my manifold is underneath the car and I can't see any hole for one to go into.
My car idles fine/drives fine. It did pass the last MOT with the previous owner, although I'm not sure whether this was with the decat or not.