Elina - //I know you wouldn't not reply (neither would I) .... but if you'd totally overlooked it say, would you pay if asked Andy .. //
If I had not replied due to an oversight, I would send a note expressing sincere apologies, but no, I would not pay.
If my friends think their money is more important than my friendship, and they don't know me any better than to know I would never be so rude as to not respond to their invitation knowingly, then I would be thinking very seriously about the future of our friendship.
So no, I would not pay!