The firmware I am using is the one pointed i3v 12" build notes:

Marlin_RUMBA_i3v_GLCDPercent at https://drive.google.com/file/d/0B80...BCdkdUaDg/view

Reading through numerous threads it appears that Marlin_Marlin_v1 at https://github.com/beckdac/Marlin
is more up to date and is the better way to go. If that is the case I'll start playing around with that version and see what happens.