Z Axis Resets 15mm Above Bed

Finally assembled my Flsun i3 Kit and after lot of false ends have got Repetier Host connecting to it.
Also corrected problem with a blank controller screen and Z axis going the wrong way.
Both were the result of wrong firmware. The supplier emailed a new version of Marlin.
The problem now is that at the start of a print the Z axis moves 15mm above the zero limit switch and begins printing in mid air!
I have checked Cura and Slic3r for any weird offsets but found nothing.
Is this a firmware issue or is it normal? What do I need to do to get it printing properly?
Have exchanged several emails with Flsun, who have always responded quickly, but I do not want to wait till Mon for next reply!

When you home the Z axis does it moves towards the Z limit switch, press it and stop? After you home it send M119 and see where it thinks it is at.

This has been the problem. It seemed to move up 15mm, every time it was started.
I think firmware was causing the weird moves. It is the Auto Level version and I was trying to rely on a Z limit switch.
But I think I have got it all working now. Printed my first part and it went well.
(The part is actually a bracket to hold the sensor as the supplied acrylic needed much modification.)
I changed over to Auto Bed levelling. Then use G28 to home the X & Y axes.
Then use G29 to Auto level. Then manual jog the Z down so nozzle is a paper thickness from bed and use that positon as Z offset.
I was also using M114 to get readout of the axes positions, but don't think I need it now with the above procedure.