Thank you Brendan.
I regularly check the fifteensquared blog, and Google results often redirect me to the relevant 225 article. Only in this case, even the keywords my memory has managed to gather up aren't enough. And checking each and every old blog articles promises to be very time-consuming I'm afraid
I was about to follow ginge's advice instead, but Mr Bartlett isn't only a great editor, he also has the reaction time of a 100m champion.
Thank you very much for your help, Steve and once again thank you brendan and ginge.
I have a full grid and part of the instruction from the extra letters which appears to contain the words "check" and "wreck". After that I am stuck as to how to identify the other phrases. Any nudge in the right direction would be greatly appreciated.