School series about a portal

I remember being shown a series at school and I really enjoyed it however all I can remember is ..

There was a young lad and girl who had some kind of portal in a box which looked like a house. Towards the end of the series the portal was massive and something was threatening to come though it.

I know its not much to go on, but its really annoying me now.

It was a series and i think ( it was a long time ago ) it was shown on bbc