Mark & Paul,
Despite my earlier post - I did not wish to pour cold water on enthusiasm - my feeling is that is a customer is going to recommend you, they will do so for reasons other than reward.
Either a genuine wish to help a friend/relative or sinply because peolple like to be the one 'in the know'.
If a reward is on offer, they will take it, and if you leave paper/card with them that has some percieved value they are less likely to loose or bin it, and here lies the true value of the reward scheme.
As an incentive to refer I think the value will be very small, so you should balance the costs against the gains.
This said, I should point out that I have never felt the need for any marketing of this kind, so my comments are untried theory.
John.