Advice is photograph the error, I can go one better and will video it. Then send unit to the manufacturer or another shop and get an estimate for repair. Then I'll have documented evidence that there is something wrong with it and I can charge Comet for this (and any other) "direct consequential cost of their failure to replace or repair".
Also emailed Tomtom, the manufacturer, and asked if they really do over the phone checks and then refuse to take the unit for checking if some kid in Comet thinks he's got the unit to work. Sent them a link to someone in Australia who had the same problem. Also mentioned I'd be happy to document the whole sorry fiasco on my website with videos demonstrating the faults should they wish to see for themselves and not rely on the over the phone tests.