I've just assembled my prusa i3 which came with an ardunio mega, ramps 1.4 and an LCD board/screen.

I have connected the wires as per the distributors instructions (motors x 5, 2x thermo couples (bed & hot end), hot end fan, hot end heater, bed heater) and power supply (+12v 11A, +12v 5A)

I have compiled and flashed the correct code activating my board type and LCD type.

However, if i plug power up only the PSU, some lights come on on the board, but the LCD is really faint. I can barely see the temperatures of the base/hot end. I can't really do anything from there.
If I connect only the USB to my PC then the LCD is fully functional, properly contrasted and I can tinker around with the settings (although without anything happening because not enough power)
If I connect both the PSU and the USB it all seems to be functional (eg I can move axis in LCD controller or via printerface) except for the extruder fan which still doesn't function.

Has anyone experienced anything similar? Is it my RAMPS board or something more sinister?

Is diode D1 installed on your RAMPS?

It looks to be. The supplier is sending me out a new board, apparently the USB only thing is a bug that can happen.