No problem, if you do end up buying make sure you get one which gives you details HRM data back, i.e. a full log and not just an average. my first one i bought whilst giving you real time (on your wrist) HR data, didn't log it as such, it just gave the average and peak at the end of the session, so not that helpful. Though, if all you want is that and it to give a calorie burn estimate, then they certainly work out a lot cheaper. I had the Suunto M5 I think for that, found it on offer at Go Outdoors at the time.
Finally, if you want to make comparative estimates, weight also plays a big factor, I am currently around 11st 6, so that may help hone your guesswork.