Outside the back of the kitchen, there's a deepish gulley into which
various drains empty.

It's about a foot deep right now. Soddit.

The bottom of the gulley opens onto the top of a drainpipe - which
clearly isn't flowing anything like properly.

I've borrowed some drain rods off a neighbour, but just can't get 'em
into the drain. I can't reach down enough to guide it manually.

Any thoughts?