Can I use the left and right channels of a stereo amp connected together into one speaker or will I damage the amp and possibly cause a small house fire?
You will damage the amp and probably cause a small house fire, Unless the amp is purposly built to operate that way, Some amp's are, but most commonly power amps.
Thanks for your answers - it's not that I only possess one speaker, it's that I live with a hi-fi-obsessed man who has approximately twenty sets of speakers and a houseful of amps and other equipment which I cannot begin to understand. I asked the question on his behalf as he has been doing a little ... experimenting with ampy, speakery, channelly things. Now, if you'll excuse me, I must just go and dial 999.