Well I returned my broken Zumo 550 and Garmin were pretty helpful and offered me a refurbished 550 for £110-ish.
The refurbished 550 turned up a week later but refused to boot up and just displayed a Zumo tm on the screen for 5 seconds before turning itself off. I phoned Garmin and they offered to collect the replacement unit and send another one. The second replacement turned up this morning and it is doing exactly the same as the first replacement: displays 'Zumo' against a blue screen and turns off after 5 seconds.
I have connected it to my computer but the computer can't find it.
I phoned Zumo and they are going to arrange free collection of the second replacement and send a third refurbished unit. I am missing something (quite likely as I am useless with technology) or have I just been unlucky with the replacement units?
I expected to take the satnav out the box and for it to start up and be ready to use like my first unit did when it was new.