automated ref number generation

hi, what im trying to achieve is generating ref numbers automatically.

I have attached an example.

In Cell A if the user choose yes it gives a ref number. The trouble im having is that say in cell A5 a Yes is choosen at a later date, it will throw the numbers out of order and the ref numbers get mixed up, there any way of stopping this from happening or any function that prevents this from happening

A pointer in the right direction would be appreciated

Sal