Bob,
why dont you tell them you have had a postponment if they did it urgently , non the wiser ,
or simply tell then you have a booking that can be put off as a regular and doesnt mind when you fit it in ,
i have even gone as far as saying ,i have a quite week next week, was due to go away but the trip was cancelled , always a way of giving the immpression you are busy when infact you may have a quite week coming up
geoff