If you have a good pharmacist, he or she can probably recommend an over-the-counter medicine for this. I think camphor was the old time remedy, and may still be used, but I'd ask at the drugstore/chemist (U.K.). If it persists, see your doctor. I think these can be minor viral infections or a dietary deficiency.
Last year had a canker sore on the roof of my mouth and the local pharmacist suggested G.U.M. Ricinol www.gumbrand.com/gum-rincinol-mouth-rinse-1770r.html
It's a rinse that applies a thin protective coating. It's available at Walmart, Walgreens, CVS...
I used to get these when I was a kid quite a bit. Whenever I did, my mom would dip a wet q-tip in some alum and hold it on the sore for about 20 seconds. Hurts like hell(and tastes even worse), but every time they were resolved within a day. I can't say I've ever gotten a canker sore from smoking though. Are you sure it's a canker sore and not a steam burn, maybe from puffin a bit to fast?