well i finally got them finished and eventually found that the best way to hold the candles in place was simply blue tac. putting coloured stones, like you find in vases with fake flowers made the lanterns very heavy. sticking a drawing pin to the bottom of the jars didn't work - i simply couldn't stick metal to glass. so in the end i opted for blue tac.
the candles seem to burn for about 5 hours, but with 10 lanterns, my 40 candles aren't going to last long! i've used 2 nights worth already, so i might have to find some more cheap candles somewhere..... i wonder if tescos' do value ones?
anyway i sent david out into the cold to take some photos once they were all lit. so here they are: