I'm searching for details of a film I saw in the UK back in 1978ish. 

The film starts with a boy talking to one of his friends on the phone
when he hears a loud noise in his bedroom. When he goes to investigate
he finds a large box. He presses a button (or something like that) and
the box opens up and there's a car and a barrier. At one end of the
barrier there is normal film, but at the other, it becomes cartoon.
The boy drives the car through the barrier and goes on lots of
adventures. When he returns, it changes back to normal film and the
box disappears. He goes back to the phone (that he hadn't hung up) and
his friend asks him where he's been for the last 5 minutes, (the boys
adventures had lasted for days).

anyone recognise this film?

Hi twistedpixel,
  
Going by this post, I am almost certain the film you recall is "The
Phantom Tollbooth" (1970), also known as "The Adventures of Milo in
the Phantom Tollbooth" :
  
"I saw this movie when I was around 7 or 8 so it had to be shown in
the early ninties. However, the movie looks like it was made around
the time of Willy Wonka and possibly by the same director or writer.
It all in color, It starts off as this young boy comes home from
school and finds a rocket or box or something in his living room. He
opens it and it becomes a little car and he goes into a spinning
portal and ends up in a cartoon world and lands in a swamp of bad
feelings. He meets this dog and the dog gets him out of the swamp, and
they later meet this king, and they have dinner and since the kid
couldn't make up his mind about what he wanted to eat, he ate his own
words, and then they go on on their journey and they meet a
grasshopper who makes them do all these tasks that would take forever
and a day. Then after something big happens, the kids learns his
lesson or something the kid wakes up in his living room on the couch
and the box or rocket flies off or shrinks or something. I saw this
movie on TV and it has been driving me nuts as to what the name is."
http://home.earthlink.net/~usondermann/found400.html
(scroll down to #351)
  
  
Another plot description:
  
"Young Milo finds a mysterious crate that has been left in his bedroom
as a gift. It opens up into a tollbooth. Milo takes a ride through it
and finds himself in a magical world. Journeying through the various
lands and encountering the strange people there, he eventually finds
himself caught in the ongoing war between the land of Dictionopolis,
whose king insists on the supremacy of words, and the land of
Digitopolis, whose king insists on the supremacy of numbers. It is up
to Milo and companions to bring back the banned Princesses of Rhyme
and Reason and unite the two kingdoms once again."
http://www.moria.co.nz/fantasy/phantomtollbooth.htm
  
  
"The Phantom Tollbooth" (1970) - http://www.imdb.com/title/tt0064806/
  
The video can be ordered from Amazon:
http://www.amazon.com/exec/obidos/redirect?tag=moneysavingga-20&path=tg/detail/-/6301971523/104-7308847-1192730?v=glance
  
  
Search terms used:
  
boy movie car "into a cartoon"
://www.google.com/search?q=boy+movie+car+"into+a+cartoon"
  
boy movie car "into a cartoon" barrier
://www.google.com/search?hl=en&lr=&ie=ISO-8859-1&q=boy+movie+car+%22into+a+cartoon%22+barrier
  
box car "into a cartoon"
://www.google.com/search?tab=gw&q=box%20car%20%22into%20a%20cartoon%22%20movies&hl=en&lr=&ie=UTF-8	Google
Search: box car "into a cartoon" movies
  
movies "turn into cartoons"
://www.google.com/search?hl=en&lr=&ie=ISO-8859-1&q=movies+%22turn+into+cartoons%22
  
portal "cartoon world" car box movie
://www.google.com/search?hl=en&lr=&ie=ISO-8859-1&q=portal+%22cartoon+world%22+car+box+movie
  
"The Phantom Tollbooth" 1970
://www.google.com/search?q=%22The+Phantom+Tollbooth%22+1970&hl=en&lr=&ie=UTF-8&start=0&sa=N
  
  
  
I trust this is the right film. If not, please request clarification.
    
Best regards,
  
Jackburton
Google Answers Researcher

Just what I was after and an answer in less than two hours. Many thanks