First, forget about the "unforgivable" in the thread title. People are mostly quite forgiving.And nothing is irreplacable, especially a 70's Martin. it's not as if it's a very old one. He'll be uspest, but maybe not as upset as you think.
   As regards the financial damages. maybe the pub landlord won't admit liability, but he (or someone else) may be of some help in having a friendly word with the lads responsible. A little advice might result in their putting their hands in their pockets to put things right.
Good luck! I've lost a few treasures in my time, and I've generally managed to forgive and forget. (After a decade or two).