At last I have Marlin running successfully on my RAMPS v1.4 hardware on my Mendel Prusa.
Marlin is a firmware 3D printer control program which lives on the microcontroller in the printer itself. Pronterface on my laptop talks to Marlin and tells Marlin what to print. Marlin is an alternative to Sprinter, which I’ve been using up til now because I hadn’t managed to get Marlin configured correctly. Now all is well.
Thank you to the folks on the MakerGear google group for advice and for assuring me it was indeed possible. It was easy enough, once I downloaded the right software and edited the right configuration options. I must have been too excited to read carefully when I first tried to get it working.