Yes, cheshut, I thought it should be Philadephus, as given by the RHS, but Chambers gives mock-orange as a tall shrub (Philadelphus commonly called syringa) of the saxifrage family with strong scented flowers.
avis50 (and the RHS) are right. Syringa and philadelphus are two quite separate species. I do wish that dictionaries would get it right. It would save people like KM a lot of agro - it's not her fault if the reference books are wrong.