MG ZS mg zs reduced power

Had the car second hand from Arnold Clark in Manchester. No running issues. In last 6-8 months intermittent reduced power when driving and glugging when driving in lower gear. Dashboard gear change signal wants me in 4th quicker than the car can speed up, has stalled on a number of occasions while driving. MPG is also shocking and very intermittently running well at distance. Local MG garage had car for over 1 week due to spark plugs and injectors needing changing under warranty. Problem re-occured a couple of weeks later, now being told clutch has gone and O2 filter needs changing. O2 filter will be done under warranty BUT clutch is not covered costing over £800. After doing some research I am concerned this is not the actual issue and don't want to waste money on something that isn't faulty. I have driven several cars and NEVER had a clutch go at all.