My opinion on this is that it's just as easy to record one channel and fix it in post, as bridge to mono. However, Shaw (the maple leaf guy), had a good point, that I had never considered.
The cheapest adaptor I could recommend would be a mono male to stereo female mini plug+jack adaptor that would bridge the outputs of the MA300 as they enter the jack on your camcorder. AFAIK, this won't create any problems, but a test would be warranted. The adaptor would probably cost your a couple of pounds. I'm not sure what it would be called, but it's a stereo to mono mini-jack converter. Someone might use one to plug stereo headphones into a mono jack.
For my money, I'd just record one channel, unless you can find an input selector in the camera setup that will allow you to select the left input for both channels.
If I need to use just one channel, I just unlink the audio channels in Final Cut, delete or mute the dead/bad one, and "Pan Center" the remaining audio track. Someone else might know a faster way.