These books retail at £5.99 each and even with Waterstones 3 for 2 you'd pay £4 each for them (I did last weekend!)
They come highly recommended by my daughter for upper KS2 i.e. 9 to 11 year olds
"A giant pack from the bestselling series which lets you be a part of history! See lava spout from the mighty volcano Vesuvius over the town of Pompeii. Watch red crosses being painted on doors in the Great Plague. Feel the mighty Titanic lurch beneath your feet. What was it like to live through such cataclysmic times? Now you neednt just wonder you can be there! Experience history in My Story, the gripping diary fiction series that tells it like it is."
There is also My Story: Slave girl for £1.99 http://clubs-shop.scholastic.co.uk/products/73414
and book covers to protect them in school bags which will take you over the £25 threshold!http://clubs-shop.scholastic.co.uk/products/70821