In fact I may have over solved it!
The picture shows the 'false' floor that sits in the cargo bay recess and slides out. The rear door will hinge down as before and magnets in the inside of the door will hold the door up against the underside of the false floor, thus creating a ramp.
When readying for take off, the false floor will slide back into the cargo hold and the same magnets will locate it to the recessed main floor. the door magnets will hold the door shut against the rear hull.
The only problem is now that the ramp looks too long! Thats about a 7 degree angle to the horizontal.
I can play with the length of the false floor to get a better look though. Just the hinge and door to redesign now.