Been standing fault in emissions system warning light

[RGJ20]

Have read previous posts on this issue and non really apply. I Was lucky enough to pick up a 2003 V70 petrol today with exceptionally low (verified) miles and a full service history, the car has done 3400 miles since Summer 2018. And less than 1000 miles since the previous MOT in Aug 19.
It runs nice and smoothly but it feels a bit sluggish on acceleration so had a quick blast at 70 on dual carriageway for 15 minutes on way home to guve it a chance to get the cat up to full temperature but the orange fault in engine emissions system light has come on. Not in limp mode (yet) im assuming that lack of use might be the problem. Well hoping it is as I bought it so could take my classic daily drive off the road for a few months for some work and dont want to be spending time and money fixing what should be the solution to a different problem. Anyone got any suggestions (I don't have access to a diagnostic computer unfortunately

[RGJ20]

Turns out the fuel has been unused in the tank for more than six months and has probably gone stale and lost a few octane. It has been driven over the last week or two but just short distances at town driving speed to get an MOT and check repairs which hasnt triggered the emissions sensors. Im guessing that with lower octane it wont burn as efficiently and produce as much bang, the engine has been trying to compensate which would explain why it felt sluggish on acceleration and the result is more CO2 and NOX from an inefficient burn exacerbated by my quick dual carriageway run it has triggered the emissions sensors. Anyone think differently? Drain the tank or dilute the third of a tank left with high octane fuel?

[Zebster]

Reading the stored diagnostic trouble code using an OBD2 code reader would be a better way to identify these sort of problems, rather than assuming it's a problem with the fuel (which I doubt). Could potentially be loads of things, but at least it isn't a diesel...