Best thing to do (saw it on the telly last week) is to put them in a barrel of water until they really stink, then water your plants with it .... superb
Naz doesnt say for what purpose you water your plants with Rhubarb solution. I found this, which indicates its for the treatment of Greenfly, Blackfly etc .
Basically you boil up a few pounds of rhubarb leaves in a few pints of water for about 15 or 20 minutes, allow to cool,
then strain the liquid into a suitable container. Dissolve some soap flakes in this liquid and use it to spray against aphids.