Many thanks for your replies. OK so I'm thinking the consensus is to forget the secret gutter and just use soakers. I wasn't entirely sure how/if the soakers were supposed to be attached to the wall or if they just remain free, as in only attached to the roof, and tuck under the cover flashing...
I'm re-roofing my lean-to workshop at the moment, and it abuts the side wall of my house. It was previously done using profiled double-roman clay tiles and they used a cover flashing for this. When I started the project I had planned on re-using the tiles and I bought a GRP abutment soaker...