If your stuck staring at them post a few pictures of the Gubbins maybe we can spot the problem and suggest a fix.

Geoff

Thanks for speedy response John. I removed both eyepieces and twiddled the focus wheel but couldn't tell what the difference was between them. They are Nikon Prostaff 10x42. Relatively inexpensive but pretty much at the top of my price range, so I'd love it if i could fix them myself.

At the moment your pretty stuck id say to get them repaired ,but if you wanted to try yourself your losing nothing ,I had the same with a pair of Zeiss bins once slowly took then apart looked down into them and saw what was different unscrewed a screw moved a prism ,put back together worked fine ,unfortunately these days most things are glued in place so not easy to get right are you sure the diopter that changes one of the eyepieces isn't stuck in the wrong position? that can be easily altered. 

Does anyone know how to fix binoculars? The left lens doesn't focus. They must have had a knock at some point and something has come loose or something. Any help would be really appreciated.