Thanks for your reply Dave. I made a mistake with the navcore version number - I was looking at the number using tt home operate my go option. The navcore I've got is 8.398.
Just tried to turn it on without power or usb lead & got the bongo noise again, so it's held some power in the battery since last night. What ever way I try to use it the screen is completely blank & black, not even a hint of any light or anything else trying to show.
I have tried it with & without the SD card & still a blank black screen.