What are the chances of you contracting cancer after smoking for 20 years but having given them up 10yrs ago?

Higher than others.: While it's true that the risk for developing cancer does decrease once you quit smoking, there is still an increased risk from the years that you were smoking that is higher than in the non-smoking population. It would be difficult, though, to put specific numbers to it since this is a multifactorial process that includes other environmental and genetic aspects as well.

Unknown: Some studies have indicated that the risk for lung cancer decreases to non-smokers level 10-15 years after one quits smoking. However, a variety of factors influence development of lung cancer and a long smoking history certainly puts you at some increased risk. I am glad you did quit smoking though.