My Nikon is back from the shop. Whilst it was there they found a secondary problem that I thought was due to 3rd party hardware.
The original USB cable that came with the camera lasted all of 12 months and 1 day. The cheap 3rd party one I bought to replace it didn't last long either. Well now it seems it wasn't the cable after all so maybe i'll download camera to computer more often now? We'll see?