If you posted 100 letters in Sussex last year, the chances are four of them would have been late.

Those are the official figures. In fact, The Argus posted 70 letters in Brighton and Hove to addresses around the city in May and found seven didn't make it on time.

The late letters, the controversial once-a-day deliveries and post office closures are all symptoms of the biggest change the service has been through in decades.

Royal Mail faces competition from other providers including, from 2007, the Dutch and German postal services, and is in the second year of a three-year plan to ensure it can survive when the service is deregulated.

Every item of mail posted in the Brighton postcode area goes to the Gatwick mail centre.

It is one of the biggest centres in the country, employing 800 people. About 320 work on the evening shift, which sends out 800,000 items of first-class mail collected from Brighton and Redhill.

The night shift receives and processes two million items of mail a night, to be delivered around Brighton and Redhill the following morning.

Royal Mail - unlike its competitors - has to adhere to a Universal Service Obligation, which means it has to deliver to every one of the UK's 27 million addresses. If it receives a letter from Brighton addressed to the Outer Hebrides, it has to make the delivery for 28p.

Royal Mail spokesman James Taylor said: "It is important people see the huge logistical challenge going on."

The Gatwick centre is the size of three football pitches and contains machinery worth millions of pounds.

Letters come into collection docks and are segregated according to their category.

Stamped mail goes through one of six integrated mail processors (IMPs), which look like huge tumble driers and are worth £3 million.

The IMPs sort parcels from letters and the addresses are read by computers.

Flat mail is processed in a 50m-long £4 million machine, which sorts up to 44,000 items per hour and also reads addresses automatically.

If a sorting machine cannot read the handwriting, it takes a digital picture which is sent immediately to "index centres".

Here, staff study the image and tell the machine where to send the mail.

Letters that these operators cannot read - because most of the address is missing or the handwriting is illegible - are sent to specialists who use databases, detective work and common sense to figure out where the letter should go.

Mr Taylor said: "We've had letters addressed to "The Vicar, that village off the M11 - the one with the pond" and sometimes we've managed to get them there.

"If it's really impossible, we have a place called the Returned Letter Centre in Belfast where people work out where letters should go if details are missing or have been damaged.

"They will try to find the sender or the recipient."

Mail centre manager Neil Molloy has come across most problems which can slow post down since the centre opened four years ago.

He said: "Some of the things that can make a difference include miscalculating the amount of mail coming from our major customers.

"We take steps to avoid that by ringing our top 30 customers every day to determine what they are sending us.

"If there were a crisis on the shift, I could move people away from second-class mail on to first class."

The centre is one of 50 across the country. The Royal Mail employs more than 200,000 people and delivers 82 million items every day - 130 million at Christmas.

It is responsible for collections from 116,000 pillar boxes and operates 14,400 post offices - even after closing 1,042 over the past two years.

According to the postal services watchdog Postwatch, Royal Mail pays £11 million a year in compensation for lost mail. Some 14.4 million letters are lost every year, of which 60 per cent are simply delivered to the wrong house.

More than 322 million items posted in 2002/03 were "undeliverable" due to being incorrectly addressed. But staff still manage to deliver three million poorly addressed items every day.

There are no agency workers at Gatwick. Managers prefer to rely on a list of 150 trained casuals, used on an ad-hoc basis.

The target for next-day post delivery is 92.5 per cent. The Royal Mail - while improving - is still falling short by about 2.5 per cent. But 96 per cent of items sent from a Brighton area postcode to another address in the area gets there the next day.

Mr Molloy and Mr Taylor find it frustrating that workers at Gatwick don't get the credit they deserve for working hard to improve the service.