Last night he turned up with his guitar again, and proceeded to murder 5 perfectly good songs.

Five? Five???

Did everyone there who wanted to sing get five songs? Did everyone there who wanted to sing even get one song?

I think the answer to this one is to be a bit more assertive with the MCing. Call on people one by one & see that everyone gets the same amount of exposure. At least that way the poorer performers don't get more exposure than the good ones.