Underground

No, I'm not also installing an underground line (that would be too much hassle if it derailed). 

A couple of areas along this bed get very wet and the clay stops it going anywhere, so, as well as getting the bulk of the clay out and adding a ton of grit I've taken the opportunity to get a bit of a land drain in.

I'm not the first - in the background is the Victorian clay one, albeit draining from a slightly higher level, so of little use now.