Having failed to get on with Memory Map I stumbled across OS Getamap last week. All I can say is it rocks and for £18 / year you cannot go wrong. I have mapped a few routes already and then you can simply export them straight onto your Garmin. The web site has a button to detect your device. Very straight forward.
Keep up the good work OS, as this is one mapping tool that is spot on for us mountain bikers!
Not sure where this leaves memory map