how can i clear a slow running external drain?

If the water is backing up at a visible gully outside (where drain pipes empty into) then its likely to have a build up of debris in the trap - cleanable by getting in there with a gloved hand and scooping the stuff out. If this isn't the case though you'll have to identify which inspection (manhole) covers the drain uses, and get in there with the drain rods. Failing that, call in a 'professional' who will do the job for you using high pressure water.

John
December 2007