I have the older version of this and it never lets me down. Great price for a GPS watch with a decent sized face.
The model with the heart rate monitor is also available for £119.95
Here's an in-depth review
A multisport, swim-proof GPS watch, ideal for triathletes and dedicated athletes. Forerunner 310XT has a 20-hour battery life and is water resistant, making it an indispensable training tool. With a high-sensitivity GPS receiver and HotFix® satellite prediction, Forerunner 310XT locates your position simply, quickly and precisely and maintains its GPS location even in heavy cover and near tall buildings and trees.