The title to an animated movie?

I do not know the name of the film, it is an older.. im guessing mid 1970's film that if i recall was done with a lot of cel work.

the synopsis is of a boy who lives in london, and befriends a giant from (somewhere else iunno) and it turns out the giant is small by giant standards, and the other giants pick on him and want to take over our world, so the boy and this (small) giant, along with the military use some kind of potion to put the bigger evil giants to sleep and then pen them up in an enormous steel lined pit.

Can anyone tell me the name of this film? i would like to watch it again.