Useful replies, thanks everyone. I decided against a heart rate monitor in the end, partly because I'd need to stop and put my glasses on to see what it says and I hate wearing them! Another example of being in middle age denial? I decided to get one of those relatively simple Cateye computers to keep track of mileages. I can see the speed OK when I'm cycling, but not the secondary info.
With heart rates, I guess I'm applying a few tips I've picked up from running and applying them in my cycling. Firstly through using a HRM on a treadmill in the gym I have a reasonable way of knowing what by heart rate is likely to be according to how out of breath I am. So, if I'm running at, say, a 6% angle briskly and panting, I know that my heart rate is probably as I high as I should go for too long (180bpm). If I can hold a conversation without too much difficulty, that's fine and probably around 150bpm and good to keep that going for as long as I want to. Having short bursts at a high rate is like interval training and this has proved really helpful in keeping in reasonable shape.
Applying this to cycling has made quite a change. I now regard going up hill as an opportunity for some interval training, rather than either avoiding them or gently winding my way up in a low gear. Most hills around here aren't too big or steep but one or two take a good 10 minutes to reach the top - so it's ideal for me ol' ticker!