hi there,
if you ask me, that's far to generous. i'd go for: three recommendations and a free clean or even 50% off. think about it, if you've got a £10 house that recommends a £4.50 front, it's gonna take three months before you make the money back. it's a good idea though, one that i was thinking of exploring. what have you got to lose? a few quid for printing etc. seems pretty low for the potential custom you could gain.
cheers, matt