Ford Fiesta engine warning light stays on or flashes over 30mph

For no apparent reason today my engine (endura-E)warning light is staying on and starts to flash if I go above/around 30mph and feels like it lacks power at higher speeds
Car has recently (2weeks) been for its MOT and completely passed no issues. am aware does doesnt normally mean anything.
Had a look at a few websites and tried to find a solution, I have changed the sparkplugs as luckily I had a spare new set.
Engine light is still coming on but car does feel a little bit better but not yet convinced its sorted.
Any ideas I could check myself am farely good at basics with car and prefer and enjoy doing things myself and not daft enough to take on something I think a garage should do.
I have thought about reseting the warning lights by disconnecting the battery to see if it comes on again but do not want to cause further issues.
really good car and seems to be in excellent condition for its age (30000miles, genuine)until this.