Hi Jerry,
"Cock-up" is from the time of early firearms where the forerunner of the rifle - more likely a blunderbuss or whatever it was called , was filled with gunpowder,and ball (or shot).
A small amount of gunpowder was placed in a compartment above the trigger mechanism , to be hit by the "hammer" which was pulled back into a "Cocked" (ready) position.The hammer would spark the powder , so that that the gunpowder in the chamber would ignite and fire the ball.
When the firearm refused to discharge, (these were early days of development don't forget) , the reason was usually that the trigger had not made the hammer fall due to a mechanical fault and therefore the "Cocked" position had remained up.
Cock-up is used when a SIMPLE action, situation , procedure, circumstance (which shouldn't REALLY happen) goes entirely wrong....

When I arrived at work I noticed something strange , the car park was empty , the whole place was quiet , Shit - it was Sunday - my day off - what a cock-up.
As for diddled , THAT has already been covered quite well BUT failing that the correct answer hasn't been yet provided I'm going for a typo. If he is in a queue for the khazi ( which usually a sit-down toilet in male-speak )perhaps "piddled" on ( or in ) me brolly would work. "Diddled me brolly" doesn't convey any meaning , perhaps he was just trying to say stole me brolly , but now we are going in for guesswork and slang is normally fairly accurate .

One thing for certain, by mistake ,accident, Empire or whatever ,the world was given a great language to have fun with, when it got English in its' many forms . Long may it live and enjoy the fun you and the thread subscribers appear to be having with it.

Cheers

Betsy.