Image caption Get behind the sofa!

Stamps celebrating the 50 years of Doctor Who are the latest in a series of special anniversary stamps. For some, the ultimate must-have is the first day cover. But who collects them?

Not everyone would equate stamp collecting with excitement. But today's Royal Mail special stamp issue is eagerly awaited in some quarters.

The company says pre-registration numbers for the set of 11 first class stamps - which feature actors who have played the role of Time Lord - were more than three times higher than usual.

For the dedicated fans, there is also a miniature sheet, a presentation pack and special postmarks on offer.

The stamps are the latest in a line celebrating particular events. So far this year, there have been stamps commemorating the 150th anniversary of the London Tube and 200 years since the publication of Jane Austen's Pride and Prejudice.

A lexicon of stamp collecting First day cover: A cover bearing stamps postmarked and sent through the post on the first day the stamps were officially authorised for use. Special postmarks have been used for this purpose in many countries since the late 1930s. Presentation pack: Souvenir containing a set of stamps and descriptive text. First issued by the British Post Office in April 1964 - it was a Shakespeare series. Miniature sheet: Contains a single stamp, block or set of stamps with wide, inscribed and/or decorative margins, issued as a commemorative souvenir. First issued in Luxembourg 1921-3, and Britain issued its first miniature sheet in 1978.

Last summer, Royal Mail rushed out stamps of all gold medal winners from Team GB and Paralympics GB, within 24 hours of each athlete's victory.

And there were stamps for the Royal Wedding, Dickens' bicentenary, and themes such as animals or space exploration.

In recent years, it has been reported that children are losing interest in stamps.

For "commercial reasons" Royal Mail says it does not release sales figures, and it is hard to know exact numbers, or what age group is buying them.

But it says more than 50,000 Doctor Who packs, or first day covers, are likely to be sold.

So how do first day covers work, and who is buying them?

Anniversary stamps tend to have two target markets - fans of the subject, in this case Doctor Who, and stamp collectors.

Different formats are available. As well as the 11 stamps of the Doctor, there is a "miniature sheet" featuring a first class Tardis stamp and four second class monsters. A presentation pack designed to appeal to fans of the TV show features all 11 Doctors, the miniature sheet and text relating the show's history.

But hardcore philatelists - literally stamp studiers - are not interested in the presentation packs. For them it is all about the first day covers.

To the uninitiated it may appear a strange obsession. It is the collection of special stamps on the day of their issue displayed on a special envelope, known as a "cover".

First day cover fans must make sure they buy them on the day. They stick them onto a special envelope and write the address they want it posted to. It can be just one stamp or the entire series.

Crucially, the envelope needs to bear a special postmark that is only available on the day. For the Doctor Who launch, Royal Mail is offering special postmarks in the hometowns of the 11 actors who have played the Doctor.

Once the handstamp has been applied, the letter can be posted to the person. Arriving in the post at the collector's house guarantees authenticity.

"It's the combination of stamp, envelope and postmark. You need the whole set - the satisfaction is having that particular postmark on that particular day," says Ian Balcombe, honorary secretary of the Association of Great Britain First Day Cover Collectors.

More from the Magazine Best known for drawing a crooked-toothed monster that meets a mouse in the woods, Axel Scheffler has used his distinctive talents to create Royal Mail's 2012 Christmas stamps. The Gruffalo illustrator shows his designs and talks about the art of stamps in this audio slideshow.

There are five Royal Mail handstamp centres around the UK but fans who cannot reach one of these centres may pre-order their stamps in advance, sending detailed instructions to the handstamping centres about how they want their cover postmarked.

"Some people will send us envelopes and specify how many millimetres from the corner they want the postmark," says Natasha Ayrvor, PR manager for Royal Mail's special stamps programme. "There's a real hardcore group of collectors out there."

Of course, there are different types of stamp collectors. Some specialise in very rare old stamps. Others, like the first day cover fans, specialise in mass issues.

Completism is a vital part of stamp collecting, according to Wayne Elliott, shop manager at Stanley Gibbons, the world's oldest stamp dealer. "Someone interested in it will feel they need to buy everything," he says.

Philip Parker, head of stamp strategy at Royal Mail, says most purchasers are adults, but many adults buy them for children.

"You also find that younger people buy stamps and then give them to their grandparents, especially if they celebrate something like WWII," he says. "For me, it's about celebrating the best of Britain."

You can follow the Magazine on Twitter and on Facebook