Does it run @ fast idle when you're driving? Or is it running @ fast idle when stationary?

If when moving try disconnecting VSS and try it.

If when stationary, wondering if ISCV is sticking , not returning to fully closed position (give it a clean)

Lambda wouldn't make it run fast.

CHT sensor 'could do' but I would expect a DTC and having worked on Focus pretty much all the time it's been in production I have never replaced a CHT sensor on a Zetec for being faulty. (first time for everything tho,eh) I would get it data-logged on IDS to check though.

Check the PCV piping and stuff under the inlet, have had them suck flat and cause running issues.

Worth spraying round the inlet with WD40/whatever to check for air leaks .

Oh, worth having a look in the throttle body to see if it's gunked up as well.