Went to see my physio regarding an ongoing injury recently and got to discussing how I run.
I'd always thought that a forefoot "good form" running style was better than heelstriking. I naturally heelstrike but am working towards midfoot/forefoot running, however, my physio suggested that often it is better just to run how you naturally rather than trying too hard to change. Is this true in other people's experience?
In a similar boat to you and I'd say go and get it checked out (something I keep meaning to do!!) and to stop running. I was on the mend and went out for a few runs that have left me back where I started (not quite as bad as it was but annoying all the same).
I don't think I'd consider 11 stone to be to skinny for your height, in fact I'd say it sounds just about perfect (I'm around 6' 1" and 11 1/2 stone so similar-ish to you). Of course it depends how (and where) you carry your weight but I'd say as long as you feel fine then you probably are!
Beats stuff is, in general, pretty overpriced for what they are (and pretty shit in some cases). A quick google will show that, for the money, you can get much better headphones.
No experience of the iSport ones.
If you run outside I think it's worth considering these http://www.aftershokz.co.uk/. I got some for Christmas and they are great, by using bone conduction they don't actually go in your ears so you can still hear cars and going's on around you.