Light Aircraft Association

I know this has done the rounds but I am reading conflicting advice on fitting a second hand mode C Transponder ..The LAA state that all new installations must be mode S full stop. However LAA leaflet TL3.03 April 2014 states .. The installation of a transponder is a useful tool in helping other air...

I would like to fit a transponder, can I legally install a mode C on a voluntary basis? Or am I now forced to go to mode S ( which I cant afford ). I appreciate there are certain mandatory requirements for certain airspace but my flying is mostly open airspace.