To mark 75 years since the first Famous Five book was published, Kate Whiting takes her family to Enid Blyton's beloved Isle of Purbeck, and finds it unchanged from the author's descriptions.

"Mother, have you heard about our summer holidays yet?" asked Julian, at the breakfast-table. "Can we go to Polzeath as usual?"

"I'm afraid not," says his mother. "They are quite full up this year."

So begins Enid Blyton's first Famous Five adventure, Five On A Treasure Island, which was published 75 years ago this September and set in Blyton's adopted home, Dorset's Isle of Purbeck.

With Cornwall off the cards, Julian, Dick and Anne's parents pack them off to Aunt Fanny, Uncle Quentin and their 'lonely' cousin Georgina, in Kirrin Bay.

As many families will be doing this summer (including my own), they leave the crowded London streets behind and soon see the 'shining blue sea, calm and smooth in the evening sun'.

Purbeck's not really an isle, more a peninsula, surrounded by water on three sides, with the island-dotted Poole Harbour to the north, the seaside town of Swanage (where Blyton used to stay in the The Grand Hotel in the 1950s) to the east, and the secluded coves and bays of Dorset's Jurassic Coast to the south.

Born in London on August 11, 1897 (she would be 120 this year), Blyton's main home from the late 1930s was Green Hedges (named by her readers in a competition) in Beaconsfield, a London commuter town bordering the Chiltern Hills, but she had a love affair with Purbeck and spent three holidays there every year for more than two decades, even buying the Isle of Purbeck Golf Club with her husband Kenneth in 1950.

With its green hills rolling like sleeping dragons, dramatic cliffs plunging down into the sea, and the ruins of Corfe Castle at its heart, it's easy to see why she kept coming back and mined the area for her famous children's books.

We take a picturesque ride in the Pullman Observation Car, right behind the engine on the impeccably well-kept Swanage Railway steam train from Swanage to Corfe Castle.

Blyton did just that in 1941, arriving at what she turned into Kirrin Castle, on Kirrin Island, owned by George's family, where the cousins and Timmy the dog discover gold in the dungeon, in Five On A Treasure Island.

There may not be any gold bullion for us to find, but our three-year-old son Ollie enjoys hunting for the six shields that are hidden around the 11th century ruins, and answering questions from the facts written on each that will reward him with a rubber Corfe Castle wristband.

A short drive from the castle is Blue Pool, a former clay pit that's naturally filled with rainwater, surrounded with scented pines and rhododendrons, and perhaps the most beautifully calm spot in the whole of Purbeck.

Blyton was a fan and described it in 1946's Five Go Off In A Caravan, where they picnic beside an 'enormous blue lake that glittered in the August sunshine'.

We wander around the 20-acre site, sipping ginger beer and watching the colour of the lake change subtly from blue to green.

"We try very hard not to change," says Miss Barnard, the 88-year-old owner of the pool and tea house, whose father bought and established it as a tourist attraction in the mid-1930s.

"We get customers who come back after 20, 30, 40 years, who say, 'We were afraid to come back in case you'd done something terrible' and they're so relieved to find we haven't."

In July 1946, aged 18, Miss Barnard began working as a waitress in the tearoom, which still has its original pale teal formica-topped tables, and still bakes sumptuous scones today.

Home for our stay is Moonfleet Manor, just to the west of Weymouth, a Georgian country pile that overlooks the iconic pebbles of Chesil Beach.

Part of the Luxury Family Hotel Group, it has its own literary connection, having inspired J Meade Falkner's 1898 tale of smuggling and shipwrecks, Moonfleet.

The hotel has its own Timmy, a tea-coloured spaniel called Snoopy, who befriends all the littler guests, and Blyton would have approved of the exotic antique decor, with menus from old cruise ships and tiger skins adorning the walls. In the hall is a chest full of buckets and spades and a rack of welly boots.

There are three swimming pools, an enormous indoor play area and a fairy door hunt to keep little ones entertained when they're not exploring Dorset - and a spa for grown-ups to relax, while the kids can be looked after in The Den creche.

From here, it's just an hour's drive through Purbeck to Poole, from where we catch the ferry to the mysterious Brownsea Island, Blyton's Whispering Island - and also described as Keep Away Island in Five Have A Mystery To Solve.

In her day, it was owned by Mary Bonham-Christie, a recluse who let it return to nature and would not tolerate visitors.

Today it's owned by the National Trust, who cleared the island of its rampant rhododendrons. As it's a perfect summer's day, we take a picnic of goat's cheese and roasted red pepper sandwiches, juicy oranges and homemade chocolate chip biscuits (ordered and brought with us from the hotel) down the steps to South Shore's sandy beach, overlooking the smaller Furzey island, where Ollie paddles and throws handfuls of seaweed into the water.

Rejuvenated, we go in search of Brownsea's most famous residents: red squirrels.

Besides the wetland and lagoon to the north, managed by the Dorset Wildlife Trust, much of the island is woodland, but no matter how much we wander around looking up into the trees, we can't see a single squirrel.

Then, just as we near the church, a flash of red darts across the path in front of us, followed by a peacock. We stop, crouch, and are joined by Hayley, a wildlife photographer with a tub of unshelled pistachios.

Soon our little squirrel is back and happily cracking into the nuts, as we watch, spellbound by its little paws and tufty red ears, like something out of a children's book.

On the drive back to our hotel, I verbally recount the whole of Five On A Treasure Island, with its shipwreck, gold and dastardly villains, to an unusually rapt Ollie, a sure sign that Blyton's power of storytelling has not waned in 75 years. And after the few days we've had here, neither has the lure of her beloved Purbeck.

How to get there

Moonfleet Manor has family rooms from £210 B&B for two adults and two children, and from £120 B&B for a standard double room. Children up to 15 stay free when staying in the parents' room. Visit moonfleetmanorhotel.co.uk

Return ticket from Swanage to Corfe Castle: £12.50 adults, £7.60 children (under 5s, free). Visit swanagerailway.co.uk

Corfe Castle entrance prices: from £9 adults, £4.50 children. Visit nationaltrust.org.uk/corfe-castle

Blue Pool is open from March until the end of November; adults £7, children under 16, free. Visit bluepooltearooms.co.uk

Brownsea Island Ferries sail from Poole Quay from March 18 to October 29 to Brownsea Island, and return through the islands if tides allow. Fares: £10.75 adults, £6.75 children. Visit brownseaislandferries.com

Brownsea Island entrance prices: from £6.75 adults, £3.35 children. Visit nationaltrust.org.uk/brownsea-island