Hi Lee sorry, was my bad, I didnt type the HR monitor name properly, its a Polar H7 bluetooth4 monitor, hence BT4, it pairs with the iphone.
I paid £50 for it, and although it is meant to pair with the iphone, being polar, it will also connect to most polar devices, and also most gym equipment! which the wahoo one doesnt.
One thing I do notice is that the first 1km or so it can be erratic if I forget to wet the contact strap first, they really dont like dry contact on skin, but theyn again they are not meant to be used that way. once its settled, it works great!
Being able to acurately track HR against pace and terrain is pretty cool if you like stuff like that, which I do 
It was so good, I bought my partner one as a present the first time she ran 1 hour continuously
Previously I had an FT4 polar, and another one which I forget the name for, they were good for monitoring, but thats it, you had to keep an eye on them all the time and it was hard to 'see' trends. I wouldnt go away from something that could chart HR, pace and terrain after using this, and when the iphone doesnt last long enough for a whole run, I will look at probably one of the garmins. bit too expensive at the moment for me tho, so will stick to the phone.