... Or, if the end of the bolt (called
carriage bolts here in the U.S.) opposite the head is accesible, you can use a pair of Vice-grip pliers to hold it while tightening with a wrench... (You might call them something different in the U.K. Here in the U.S. they look like this
http://www.all-spec.com/products/VG7WR.html?ut m_source=yahooshopping&utm_medium=feed ). This will probably damage the thresds slightly, so don't screw the adjustment on the Vice-grips too tightly.
Another "fix" that works pretty well, is to cut a small washer of rubber out of any available material (I use an old inner tube from a tire (tyre?) or even a small rubber washer or "O" ring, fitting it snugly over the square shoulder of the bolt. This gives enough friction on the sides of the stripped out hole to hold it for tightening. Helps to thoroughly lubricate the bolt and nut if replacing the original... better yet use a new replacement... Best of luck!
(I like the idea of cutting a screw slot in the head, but sometimes the head isn't accessible when in it's intended position.)