When I start composing an SMS message on my Samsung Galaxy S2, if I pause for a couple of minutes the SMS gets sent automatically, even though the message hasn't been finished (and I haven't hit the "send" button). Is there some way I can stop this happening, please?
You could start off by checking if this is a problem with the Messaging app itself. Install an alternative SMS app (I use GO SMS Pro myself).
If all's fine with the alternative app, and you don't mind using it (they're much more powerful anyway), problem solved. Otherwise, you could try out what patchie is suggesting.
I would re install Android to the newest version. Either do it yourself, or go to a phone shop and pay 10$ or so for them to reinstall your phone.
If this solves your problem, it was a cheap fix. If it didn't work, you can go back to the phone shop where you bought the phone and make them fix it in guarantee.
If you want to try to download and install an awesome rom for your phone, you should read this: http://www.cyanogenmod.com/devices/samsung-galaxy-s2