I only past my test 6 months ago and my 1983 Austin Mini was my first car so I know very little and I'm new to this so bear with me please!

Ok, my mini has been running fine since I got her but recently she's been misbehaving. When I accelerate it's VERY jerky and very slow (1-5 mph even after a few minutes with my foot to the floor) and I think she's going to stall but somehow doesn't, I start her using the choke and when I shove the choke back in it helps a tiny bit. She does this 'jerky' and loss of power from starting but also once she's been going a while and has warmed up (this week I had been on the motorway for about 30 mins and she slowed right down to 30mph and started jerking for no apparent reason!).
I don't drive every day and I don't have a garage so could it be the cold weather and the rain? Also this started happening the day after I put tesco's 99 petrol in instead of the 95 and there still might be a tiny bit left in the engine so could this be the cause?
Basically what I'm saying is I know Mini's are notoriously stroppy in the cold and wet (who isn't!) but I don't know if this is normal behaviour and once it warms up all will be ok again, or if I screwed things up with the petrol incident, or if it's something else entirely and I need to get her checked out....
Almost forgot, today when I'd let the engine warm up I pulled off and stalled, then stalled again then she did the jerky thing then twice a big puff of dark grey smoke popped out the exhaust! This was new!! Does that shed light on the problem?
Does she need a blanket and a rain cover or is a trip to the garage in order?
Any help/advice would be much appreciated!!!
Thanks
