I believe the hot air blowing in is the control valve knackered I would take both lamber sensors out dobt touch them as grease in your fingers can damage them Shen up to temp , leave them to soak in petrol tgen wipe with a clean rag , Lamber sensors alter the running of the engining to make it run as clean as possible , this is why there is normally a lamber sensor before and after the cat , if that doesn't work they will only be cheap on e bay last time I fitted one it was a 22mm nut , look for the 1st in the manifold and 2nd in centre pipe . I would do this and the coolant 2 cheap flixs and see what it runs like
Lee, Welcome to the club , you will find everyone friendly on here . The X type is a cracking car, I'm sure you will enjoy it . Don't worry about MPG , let others do that for you, and you do get used to it , and it's better than some VW dinky toy run on devil's fuel ! ATB, Trevor