Vauxhall Astra misfiring, catalytic light on

Put petrol in car, drove about half mile and started juddering, misfiring, losing power, cut out. RAC came said could be coil pack. Took to garage where it had passed MOT just the night before. They said it was coil pack, replaced it at cost of £106. Mechanic started engine still misfiring, he took out spark plug(s) not sure how many cleaned them, then it was ok. Drove home about 1 mile and was fine. Drove about over the next week, not far, and seemd to judder now and again. finally drove home and car was juddering, losing power and misfiring, light came on again. Car has been sat idle for 10 days now waiting for garage to reopen after extended bank holiday break. Any ideas as what now could need fixing or replacing. I am ringing garage in morning damanding they look at it again but I am reluctant to spend any more money on things that probably don't need replacing like the coil pack. Also could the petrol be to blame? it was unleaded and I am still using the same petrol when all this started.