Brithunter: when you chambered the first round did the firing pin follow the bolt home or stay locked to the rear?
How did you finally clear the problem?
Do you have two 'faces' on your cocking piece - you said you polished the face on the cocking piece and shortened the sear height - is it possible that when cocking the rifle the sear may have slipped past the sear face into the 2nd notch (if there is one)??
Could it be that even though you maintain your rifles in sqeaky clean absolutely proper British condition (compliment here) that your reduced sear spring may have not functioned as expected and did not tension the sear or sear release into place?
??
I have a Husky M38 that has been sporterized but I pulled the wing safety and installed a Bold Trigger rather than to polish the sear face and reduce the # of coils on the sear spring because I feel that normal wear will get you to the same place (trigger pull and let off), eventually, and that you may be setting yourself up for problems later. As a parallel, I purchase new spring sets for one of my Smith and Wesson revolvers because years ago when I just used to cut the coils off the rebound spring and/or the hammer return spring (on some models), I would occasionally get failures to fire and sometimes even a cylinder hang-up.
Try pulling the sear spring and see if you can duplicate the problem and if that doesn't work then run the quesation past the gunnut69, the Moderator of the Gunsmithing Forum. HTH.