Does this cable need burying?

[youngplug]

I need to run a length of SWA along the back wall of the house. The route crosses the back door. It won't be very pretty to go up and over it. However, there is a recessed gap between the door threshold and the patio slab. Obviously it's in an area of foot traffic, but it would be visible and couldn't actually be trodden on due to the recess. Is it acceptable to lay the cable in this recess? Or is necessary to bury it? And if I bury it what would I have to do? I know that normally buried cable needs to be at 500mm with marker tape, but could I do it less than that if, say, I cemented over it? Or, could I lift the slab and lay it under there, and if so could I do it at less than 500mm?

Thanks for your advice