Misfire Cylinder 4 - Fiesta 1.4 Petrol MK6

Okay, long long story I'll try and make it shorter! Have a MK6 Petrol Fiesta 1.4 with a misfire on cylinder 4. The misfire appeared one afternoon on a dry day after a short 30 min drive on the motorway. It began with a very minor misfire just once or twice for a mile or so and then progressed to a full misfire with EML illuminated and struggling for power across the rev range but especially under acceleration.

Code reader diagnosed this as P030 'Cylinder 4 misfire detected'. No ther codes present.

Replaced:

All 4 spark plugs
Coil Pack
HT Leads

Only difference was a very mild improvement at initial start up followed by a repeat of the misfire and EML.

I then:

Swapped injector No.4 with Injector No.3 to see if the misfire moved - it didn't.
Replaced injector no.4 with new (main dealer) injector...still same problem
Injector Cleaner....same problem

At this point I gave in and took it to my local Garage. £45 lighter they called to say the car was fixed. On inspection, no more misfire, no eml. Apparently he'd checked the wiring at No.4 injector and found no obvious fault, reconnected and the fault was gone. Off I drove and it remained great for nearly 3 minutes and then returned once again.

I've read a few people suggesting to replace the Lambda Sensor but I see no reason to? It's on the exhaust side of things, is not showing up on code reader (would normally be 'Heater Circuit Bank 1 Sensor 1' if it was).

It' also been suggested to me (Garage again) that the ECU could need a 'reset'???? What's this involve and where is it???

Any other suggestions welcome!!!!