I have STI Legacy 2006.

Recently the Engine warning light has come, after engine has warmed up and traveling as speed and then run on the over run, as soon as I slow down below 40 mph or come of duel carriageway the light goes off. Car runs very well would not notice any difference in performance.

have changed the Mass flow meter, and both 02 sensors pre & post, checked for vacuum leaks all seems good. Air cleaner also good.

Any ideals would appreciate your thoughts.

Any one know a good Subaru expert in the Guildford/Farnham area.

I think thy both relate to air flow, O2 sensor, clogged air or fuel filter.  There is a stack of stuff to clear these faults if you Google the fault codes.

Ok thanks great 👍 I will check them out.