This is a fantastic script!
I too had problems with the popup OS map not appearing - managed to fix this by editing the script. (You can do this by clicking on... Tools -> Manage User Scripts -> Geocaching Map Linker -> Edit - you might need to set up Windows so that .JS files are associated with notepad first.)
I changed the following line:
function _popupOSMap(gridref)
to:
window._popupOSMap=function (gridref)
(After reading the advice at this link).
It works fine now!
I can also agree with Team Paradise, for 12.95 the lynks cables eD1 is great value compared to the official Garmin ones. Had it for about a year now, works perfectly.