This is correct: 31 January 2015. See http://www.flyertalk.com/forum/briti...an-2015-a.html.

The last codes were issued at the end of 2010, valid for bookings in 2011. Those codes are the ones that still work, so if the OP still has those codes and they worked in 2011, then they should still work now.

However, as gms points out, the scheme doesn't allow the OP to use the discount for bookings on which the shareholder is not travelling.