In reply to Stuart (aka brt):
Thought I'd reply and complete the circle.
Firstly thanks for all the pointers. Good food for thought. As it was I managed to resurrect the 310xt and bodged something that might keep out a small amount of moisture. Under a rain jacket it'll probably be OK. On a bicycle? Nah. So it's sidelined for running only.
For the cycling I opted for a Garmin 130 which for the dozen or so rides I've been on, seems like it does everything I need. The nav system is basic (as I knew it would be) but easy to follow.
The data fields are plentiful and easy to arrange.
Did a couple of back to back five hour rides without recharging and the battery had a little bit of juice left.
Got BC discount plus Quidco at Halfords and ran it through one of those money saving apps too, so about £20 off ticket price.
So far, so happy. The suggestion about a discounted replacement through Garmin will be something I look into as I'll very likely end up having to get a dedicated running watch soon.
Thanks all.