Also the ground has been much heavier this winter causing xc times in general to be slower.
Suzanne was it your first half marathon? Your opening post suggests that your longest race beforehand was 10k. Have you fully evaluated your race - this could include how you felt on the day, weather conditions, terrain and your consistency of running (e.g. did you start too quickly), any or a combination of these could've led to your disappointment.
In answer to your initial question, treadmills can help train your cadence for certain paces but be careful about becoming reliant of them in lieu of running outdoors; your body still needs to be conditioned to running up & downhill, turnings, weather, surface etc. so to mix it up a bit between indoor & outdoor running should be a better approach.
I agree with Coughie & Max's Mum that speedwork / intervals will help. Some good sessions can be found under the training tab above.
A sub 2hr half is a fraction over 9 minute miling so it should also benefit you if you try to hit this pace in some of your shorter distance (working up to 5 miles) training runs.
Stick with it, I'm sure it will come. Good luck