Dug my very small (about 8' x 4' x 18") about a year ago, put some
frogspawn in it, it hatched, infested the neighbours gardens (but they
don't have slugs now)

was probably toads....was the spawn in strings( toads) or one big mass of
jelly(frogs)? If you only saw one it probably didn't lay any eggs mine breed
over the water then lay the eggs. I am sure it will get more dragonflies next
year. Dragonflies are wonderful they eat mosquitos. Jerri

http://www.fringeweb.com/Ponds/JerrisPond