Handset Registration?

I have been given another Synergy 2110 handset by a friend. I already have one so I need to add this handset to my existing base station. I have lost the instructions and when I try to get them from BT.com I find that I just get a http://page not found error. Can anyone tell me how to register additional handsets on an existing bas station (with 3 handsets already on it)? or alternatively, can anyone point me in the direction of the BT pdf instructions on another site other than BT's which will load.

Thanks for your help