The comments above re: data analysis are very valid.
Yes you can get a heart rate monitor for £15, which will give you an instant readout, maximum and average but with no data storage / transfer unless you write it down. I started off with one like that
After a while I bought a Garmin Forerunner 305, the big advantages for me were data transfer to training centre software that comes in the box and that it has GPS so you can plot heart rate against elevation against speed against distance. Another important feature for me was you can set a maximum heart rate alarm, so if you are overdoing it you know before you collapse.
The software is so flexible you can overlay different colour graphs for all these factors and also compare a route from last month.
It also links to Google Earth with your track highlighted.
The Forerunner has now been replaced by a newer model but they are still available at under £100 from Amazon.