Ah the Pantry Chicken is that strange little beast that lurks in the larder, but whose existence it is impossible to prove. It is responsible for mysterious crumbs where there used to be cakes. For scuffling, scuttling noises heard on opening the door suddenly. For any strange noises heard through the closed door. And most notably for the fact that eggs - that turn out to be hopelessly addled - turn up in the most unlikely places.
The chicken itself of course is mythical, but it is used to explain any of the above (and many other) inexplicable phenomena involving this store room.

I once overheard an elderly pair of ladies discussing Pimlico pasties - any ideas ?

It's difficult to tell the context from your question, but if the ladies seemed to be the kind of elderly ladies who enjoy their food, then they were probably referring to savoury filled pastry snacks. Pimlico pasties, in particular, are characterised by a generous admixture of horsemeat (Pimlico being a racecourse in Maryland, USA). On the other hand, if the ladies still looked in fine fettle, slim and a little cheeky, they may have been referring to that variation on the glittery stick-on pastie used by strip artistes as a rather vestigial modesty aid - a tip of the hat, if you like, to covering at least one goose pimple - the Pimlico Pastie, made entirely of cellophane.

Stiff Gallop?

I think you misspelt it. A Stiff Gallup is a election/poll/whatever in which all of the choices are equal given the margin of error. So named because it was said that deadlocked votes were the only means of excitement for Mr Gallup.

Tuna Harbour

The semi-mythical Tuna Harbour is a massive 'C'-shaped formation of rocks, just below the surface of the Pacific Ocean, which is possibly the remnants of a long-extinct underwater volcano. As such, the composition of the rocks are such that marine life thrives there, and hordes of tuna descend upon the place daily to feed. Canny Hawaiian fishermen have sussed this out, and are known to make frequent nocturnal trips to Tuna Harbour, where a little effort guarantees great satisfaction.

I heard that a friend of mine was caught polishing the yucca. Is this serious? (And do I win £10?)

Very serious. Though it sounds like simple hanky panky (if you know what I mean), it is, in fact, much more serious. It concerns the fraudulent filling in of email addresses when requested by websites. This may seem pretty innocuous, until you realise what damage is done by perpetuating false email addresses. Think of the vast quantities of important advertising email that is sent to bogus recipients. Think of the plight of African victims of oppression who have several million dollars to move to the West, and can't find anyone trustworthy to help. Think of all the poor devils who need parts of their bodies enlarged, or access to life-saving drugs like Viagra. And think how email-spoofing behaviour is harming all these wonderful enterprises. The name is derived from the unfortunate Yukka Polish Inc, which pre-dated the internet. They send out their adverts in these handy reply-paid inserts that you just love to find in magazines, but to which some twisted pervert of a DJ on The Light Programme of the BBC took an unaccountable dislike. He recommended that listeners fill in the reply with a bogus name and address and send it back. The resulting postage-paid bill would have sunk the company anyway, had not the weight of accumulated mailbags not caused the company HQ (a portakabin) to slide into Portsmouth docks with the loss of all hands.

Try something less vicious, like the Belfast Sandwich

What did I say that stopped the game dead?

Dunno, Gil - I thought it would be cheeky if I posted again so soon on this one!