Try adverts.ie or done deal Graham. If you're gonna go for one go for the edge 500. A 705 might seem cheap in comparison but it's sorta out of date now not that I know f-all about them. Just been lookin into getting one myself so have done a bit of checking up on line. CRC are doin them at the min for €190 excl HR or cadence sensor... And at the pace you travel you'd have no need for a HR or cadence sensor anyway!
705 shows maps on the units screen which can be consulted while riding, used for navigation, good for touring for example..... for off road MTB'ing I assume its not a great help as the routes will not be on any maps you can download to the unit... I could be wrong here of course.
500 does not show maps on the units screen... but shows all you need to know really.. screens can be set up to show speed, HR, time, duration of spin, cadence (if you have a sensor)..
Best of all you can upload to Strava or similar and track your progress and see how you compare against the good guys... reasonably accurate... and heart rate.. yuo can scare the sh*ite out of yourself when you see what you do to yourself.... a man of your age and all that :-)
I had 305 and 705, both do more that I need. 500 and the new ones are just better designed, smaller, lighter and the mount is safer.
They all offer the same functionality, just in different boxes.