Tara Connor spends her working week inside a cage, struggling with stuffed owls, wads of banknotes, engagement rings and exploding sausages. Before you leap mistakenly to the conclusion that Connor is a performer with a travelling circus, consider this. Six hundred thousand items of post go astray every week through no fault of the Royal Mail. At this time of year the figure rises to a million.

Connor is one of 200 letter detectives employed by the Royal Mail in its National Return Centre in Belfast. Or as she and her colleagues describe themselves: “We are the Specials.” Her mission is to reunite an eclectic mountain of items with their senders when the system breaks down - usually as a result of the failure to address and label letters and parcels properly.

She's losing sleep over a little old lady and the two-litre plastic bottles of milk that she posts to her grandson every fortnight and that always end up inside the Cage - Connor's top-security workplace - swollen to the size and shape of rugby balls. The Riddle of the Undrunk Milk, of which more later, is proving a tough one to crack.

But first, what does it take to be a letter detective? Equipment: one Stanley Knife, one internet account (because it's true, Google really is a detective's best informer) and a keen nose. “There's no formal special training, it's more to do with attitude,” says Connor, who is 35 and who has been inside the Cage for much of the past eight years. “Sometimes we get fed bananas,” she wisecracks as we are led through the barrage of security and CCTV that surrounds the only unit in the Royal Mail's UK network where staff are permitted to open parcels and letters.

Bins the size of shire horses are stuffed with loot; piles of Jeffrey Archer paperbacks are here, Schiphol airport jigsaw puzzles there and neon children's clothes everywhere else. Shelves are starved of space by car collectibles and picture-vinyl records. A grey plastic bag, pretending it's not there, contains Rampant Rabbits.

“You do get a glimpse into people's lives,” says Connor. “We get the strangest things in here. A lot of live pet food. We get quite a lot of crickets and meal worms and locusts for people who order them online for their pets. We had a bird-eating spider that came in here once. Luckily it was stuffed, but there's one of the guys who's absolutely petrified of spiders, bless him, so I felt quite sorry for him.

“Usually we're able to get the live things back to the company that sent them the next day so they are turned around quickly and they live to fight another day, so to speak. Until they are re-sent to another lizard or snake.

“One day the meal worms escaped all over a trolley. Our manager fortunately took the hit on that and put them all back in the parcel they came in. We sent him in, he's paid more than us.” Time, perhaps, for an uplifting anecdote. “We had an engagement ring that was sent to one of our clients from a jewellery shop. He was whisking her away to the Eiffel Tower to propose and unfortunately the item was incompletely addressed by the jewellery company, so it ended up in here. The jewellery company rang us when it didn't turn up. We located it and got it to him just in time for the flights. We loved it, getting that back to him. He was a very happy boy and the ring was beautiful. She was a very lucky girl.”

At the other end of the matrimonial scale is an experience that still sends a dark cloud scudding across Connor's brow. “We had an item this time last year that a father was sending his children who had moved to Canada. The parents had split up. She'd moved to Canada with the kids and he'd sent these two beautiful boxes of stuff. The ex-wife refused them, so they came into our centre here and you thought to yourself, those poor kids were wanting something from their daddy on Christmas morning and they didn't get it. He'd wrapped everything inside it beautifully, with a message saying ‘I miss you boys, and here's a picture of myself' and stuff.

“We didn't manage to find a return address and it went into storage. He got in touch to claim it. Subsequently we found out that he'd re-sent it and the wife graciously accepted the father's gifts for her children. They didn't get the presents until about March but at least they got them, so it did have a happier outcome.”

On the whole there is little here which would inspire the potentially light-fingered to indulge in a spot of “shrinkage” - not that aspersions are to be cast on the morals of the Specials, especially when their every movement is being monitored by silently winking cameras.

But you might be surprised to hear of the amount of money that gets sent through the post. A postman was jailed in 2005 for six and a half years after he was exposed as the mastermind of a £20 million fraud involving credit cards and chequebooks stolen from the Golders Green sorting office in North London.

The Royal Mail has tightened its security since then, and the impression is quickly formed by a tour inside the Cage that the workers here are all made of the right stuff. If money lands on their desks they cannot wait to get it back to its rightful owner, as in the case of the man who accidentally dropped £11,000 in an unmarked envelope into a postbox.

“The guy was going to a night safe with it and he didn't realise he'd posted the money with some letters until he got home. “His company phoned customer services and it came to us. Any loose cash that's found in the post comes to us and we log it all on computers so people can get it back. I'd just say please send anything with cash in it as Special Delivery.”

As Connor and her fellow letter detectives prepare for the post-Christmas onslaught of rank sides of smoked salmon and curling, leathered kippers her thoughts keep turning back to a special sender and the case she has not yet cracked.

“There is an elderly person, bless her, who is sending two litres of milk to one of her family who is a student. It happens every two weeks. So we get this box every fortnight with two two-litre bottles of milk inside it.” Connor knows the recipient is a university student, but cannot reveal further details because of the wretched bureaucratic bits of the Data Protection Act.

“We genuinely have tried to find this person, just to tell her to stop sending it or to tell her to phone her grandson and tell him to go and pick them up. She's spending an absolute fortune on special delivery postage to get them to him for the next day.” Reader, keep your eyes peeled for a calcium-deficient student and help Tara Connor, Letter Detective, to sleep more easily at night.

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