Unwanted guests, undeserved accusations and flap cracks

I must apologise unreservedly for an horrendous slander further down this blog. I implied, nay stated, that Prince Flufflupagus von Tuftylumps, aka Ringo, aka That Darn Cat, was annoying Prince Charming and me at night with demands for food, eating us out of house and home and had broken through the door of his cat flap.
Well. It's much more sinister than that. We had musjudged the poor beast, as we discovered last weekend when the realisation began to dawn that We Were Not Alone In The Living Room. A very, very ugly and frightened looking black cat was lurking behind the sofa.
We attempted to coax it out, but it bolted past us and ran upstairs to the spare room, apparently no stranger to the layout of the house. It flattened its ears and hissed at us, but we eventually chased it out and got it safely out of the cat flap.
We soon regretted out sympathetic treatment of the beast, however, when we noticed the fragrance of stale cat piss pervading the living room, and it began to occur to us that Ringo had perhaps not been eating all that food himself - indeed, that his frantic early-hours clawing may have been because he was frightened, not greedy. How guilty we now feel to have reacted angrily in the middle of the night. Cat-loving readers, fear not, however, he has been well and truly rewarded with extra Kitbits and cuddles, and I think he's starting to forgive us.
The cat flap has now been completely pulled off, apparently by the impostor, so is currently blocked with a pile of PC's toolboxes which not even a nasty black fiend can get through. Ringo is once again venturing outside for short periods of supervised play, although a little nervous about going out on his own, and Prince Charming braved the unenviable task of cleaning the wee off all the furniture (luckily, we haven't got a proper sofa yet otherwise it would have been ruined).
The Evil Black One has only been sighted once since, and we are going to pursue a strategy of being so scarey and horrible to it that it won't want to come back in the house again. I am going to get one of those magnetic cat flaps for Ringo, but I'm not sure if he will like having the magnet thing on his collar. We shall see. He is a big fat lump though (not all the extra food appears to have been filched by the intruder) so he needs to get outside for some exercise so I may not allow him to be fussy about the collar.
Mind you, of course the first thing he did when he went outside was eat loads of grass and then come indoors again to sick up a nice furball on the bathroom mat, but hey ho, that's the joy of pet ownership.