Print starts in mid-air

[Billzilla]

Hi there.
I've had an UP 3D printer for a couple of years and really like it but it was time to get a unit with a larger print volume and having twin extruders is quite handy as well, so I got a Flashforge machine a week ago.
The test print of the supplied cube-like thing went perfectly, but when I try to print something of my own design the printer starts to squirt out a little plastic in the FL corner of the platform, then zips diagonally over to the RR corner, then goes off the edge and starts extruding into mid-air.
I've set the object in the centre of the platform, what have I missed?