I'm not sure its that great an idea tbh, youd probably need to check that the sensors work with both the app you want and your handset and it'll presumably eat more battery power. If you want to be able to see your stats while your riding the screens going to have to be on and from experience most smartphone screens bleed the battery dry quite quickly, you'll also need a decent bike mount that wont drop your phone on the road or let in rain or even damp/foggy air. Maybe turn the gps app on and turn the screen on permanently if you can, see how much battery you eat per hour to see if the battery can keep up.
Given you're looking buying ANT+ adapter, HRM strap, bike sensors, probably even parts of £100, you might be better looking at getting a Garmin Edge/Forerunner in the <£200 range, which would probably do the job better. More accurate gps, much much better battery life etc.
Smartphone tracking is awesome on a basic level, but it does have its limitations IMHO.
