Engine management light comes on for no reason

Could check the easiest things first like the plugs and leads , may be breaking down intermittently under load and triggering the light , especially cyl number four .

The 1st time it happened on the way to Bristol street motors for a service they checked system done full service charged £50 to switch off the warning light 2months later light back on. Recently took it to another ford specialist garage done dianostic check replaced ignition coil and cylinder 4 ignition lead 2 days a go now the light us back on code P0304

If the plugs were replaced in the service , they should be itemised as parts in your invoice if so . If I had a car in for a full service customers would not have been charged £50 to switch off the warning light , it would be FOC . If the plugs have been replaced , it could be an intermittent injector fault which you might want to try using one of the injector cleaners on the market which you just add to the fuel Other possible cause , low cylinder compression ,

Thanks i must add there seems to be no effects to the sound or performance to the engine ?? another possibility i have heard of people experience this say the cause has been related to the fuel tank petrol cap not being secure mine is the spring loaded type you have to push with the petrol nozzle to access and it seem secure??