The 3 stone you lost, does that mean you are currently 260 or are closer to 220 now?
Not trying to discourage you but I'd try to lose more before trying to get pregnant if you are still 260. A friend of mine was overweight and got pregnant even though her doctor told her she needed to lose weight. Her doctor then told her to start walking and try not to gain too much weight during her pregnancy. She didn't listen and gained 50 lbs while pregnant. When her daughter was born she had so many complications by the time she was 1 she had seven surgeries. Her doctor told her it was due to her high weight pregnancy (I think she topped 300 lbs at the end).
If your period is sporadic and you are trying to get pregnant you need to tell your doctor. You may have issues trying to get pregnant due to your weight, and having few periods might be a sign of PCOS (polycyst ovarian syndrome), which can make it harder to conceive and if so it's even MORE imperative you lose weight to conceive.
Here's a great link about what PCOS is:
http://womenshealth.gov/faq/polycyst...y-syndrome.cfm