I don't suspect roots in the drain itself but in the rubble pit the pipe empties into. That's about a cubic metre of soil that's been removed and then filled with large rubble and then smaller at the top and finally a layer of soil, and in our case, then pavers. This pit drains into a trench. It's basically only got shower water and washing machine water going into it, what we call grey water over here, so it just breaks down in the soil.
Anyway, I've got some guys with cameras and stuff coming later on today to have a look and see what they can see.
The thing that's irritating me today is the headache I've had since I went to bed last night. It's still with me.