the magic shop

i read a book called 'the magic shop' about 15 years ago when i was at primary school. i didnt understand much of it at all at the time but parts of it have always stuck in my mind. now i am a little older i would like to find this book and re-read it.
unfortunately i dont remember the authors name and with such a title, websearches yield lots of unuseful results. a search on amazon books also gave lots of modern books with similar titles. h.g. wells wrote a short story called 'the magic shop' but i dont think that it is the book i am looking for.
i was wondering if anyone recognises the books name. it would have to be at least 15 years old but probably older, as i read it in my school library. i rememberthinking it was the sort of book that looked too old and boring for children to want to read.
i have few memories of the actual text. i think there were two children who went into a magic shop and found a secret portal to a new world. in this world they entered a cavern in a cliff-face and found prisoners who were mesmerised by television. i remember the book described this television type thing but never used the word 'television'. past this point i dont remember much more than a showdown in a chasm area of the cavern very near the end of the book.
i would love it if someone recognises this book and could help me to find it. for me it represents a missing piece of my childhood.
with regards,
tommy.