Try to stick to 1200 cals a day.
You can eat HUGE salads for lunch : lettuce, cucumber, tomatoes, red onion, spring onion, a big pile of chicken, a boiled egg and some fat free dressing. Or, if you don't have time, grab a Mug Shot.
For treats, substitute crisps with Ringo's or Maize snacks.
Instead of chocolate, get fat free mouse (99 cals) or mini cornettos (also about 99 cals)
If you do this, and walk for about 3 miles a day (not necessarily all at once), I guarantee you that the weight will fall off, because I've done it myself.
I think you could drop a stone a month doing this. So, by Christmas you could be 15 stone.
Just always have low cal snacks to hand. I never went hungry when I did it. It sounds hard, but once you see the weight falling off, you will find your resolve increasing.
Good luck! 