Hiya, I have gone through similar things - I have so far lost 4 stone in a similar way.
Couple of tips which work for me.
If I have been really good for about 4 weeks and notice that I have plateaued, I have a f*** it day and eat lots of crap food. This kick starts the weight loss again. Not sure why, but it works every time.
I got 'stuck' at 14st 6 - I go to the gym at least 4 times a week and go on cross trainer for just over an hour at a time. I like to get my heart rate around 150 - but in the last few months have found on the cross trainer that I now struggle to get it over 135.
I have now started to cycle to work - and that helps a lot. My heart rate is pretty much at 150-160 for the duration of the ride.
I would definitely measure your heat rate (get a monitor
http://www.hotukdeals.com/item/423781/garmin-f orerunner-50-sports-watch-w/ if you can as the gym ones tend not be so accurate)