CEL on - no noticeable performance issues...... but....

[RPB]

Hey all knowledgable folk out there. Over the last 10 months my 2011 S3 CEL came on and then went off after two days. It has done this twice. Then last week the CEL came on again, having been off for months, and this time it is staying on (not blinking). The car drives ok and I cannot feel any difference yet in how it is behaving (I don't accelerate hard much as most of my driving now is on Nottm ring road, full of cameras), so kind of wondering what it could be.
The car is booked in Weds for service and MOT anyway so they'll check it out but it will fail that MOT already with the CEL on.
So my stupid question is this..... I know there could be loads of reasons yet but are there any "usual suspects" for the CEL on my S3 to come on?
Thanks folks.

 

[NHN]

Need to log it to see why it's on.

Could be multitude of things, check the cam follower is ok for 1, as if damaged can cause fuel pressure issues, but if the cars running ok it's probably not that.