His bouffant hairstyle became a familiar sight on Horseguards Parade, where William Tallon walked the Queen Mother’s corgis.

Over half a century of loyal service, Mr Tallon, who was known as Backstairs Billy, amassed a treasure trove of royal memorabilia.

A year after his death at 72, his collection is expected to fetch more than £500,000, considerably more than what he would have earned during his 52 years as the Queen Mother’s favourite page.

The hoard in his one-bedroom grace-and-favour apartment provided by the Duchy of Cornwall includes notes and photographs from leading members of the Royal Family, and souvenirs of his long service, including a gold box engraved with the initials ER, given to him by the Queen as a memento of his boss after her death in 2002.

Among the highlights is a handwritten note from the Queen Mother stating: “I think I will take two small bottles of Dubonnet and gin with me this morning, in case it is needed.”

There is also a photograph of Mr Tallon dancing with his employer and another of the royal corgis sitting on the throne.

The 700 lots give a unique insight into royal life, include unseen and autographed photos of senior royalty, personal gifts, antiques and dozens of handwritten notes.

Mr Tallon, whose official title was Steward and Page of the Backstairs, was the son of a Coventry shopkeeper. He built up his collection with the assistance of his long-term partner, Reg Wilcock, the Queen Mother’s Deputy Steward and Page of the Presence.

He developed a fine eye for antiques and works of art by, among others, Sir Hugh Casson, Augustus John and Edward Seago.

But it is the royal memorabilia that is expected to attract the greatest interest when it is sold by the Colchester-based auctioneers Reeman Dansie on Saturday.

James Grinter, the auctioneer, said: “We’ve had worldwide interest and we’ve even got collectors travelling from America especially for the sale.”

A two-page letter from Princess Diana written on the first day of her honeymoon, thanking the pair for filling her Clarence House room with flowers, is estimated to fetch between £1,000 and £1,500.

After Prince William’s birth, the Princess wrote again saying: “We are not sure at the moment what has hit us, except a very strong pair of lungs! Both parents are making little sense, we just seem to spend most of our time gazing at this tiny person.”

Mr Tallon was so proud of his royal service that he never gave any memento away.