Pensions contributions due by ?

On a reasonably regular basis I am asked by clients to establish the amount of pension contribution they can make in order to maximise the tax benefit, which I have no problem in calculating.The next question is invariably,"When do I need to make that payment into my pension by?"
My answer is that technically the amount can be paid to the scheme administrator by 5th April 2001 if you wish it to be treated as having been made in the year 1999/2000, although if you wish the payment due on / by 31st January 2001 to be reduced you obviously need to make the payment before that date.
Am I correct? Or am I badly sadly wrong? I just cant seem to find this written down anywhere.
Any help would be very gratefully received.
Neil Brookes