So. Spent most of the morning furiously trying to work out why I wasn't getting a reasonable response from my Rest Service "RS", calling it from some other system. Only got something vague along the lines of "value must not be null". Both systems are set up on my local machine, IIS bindings set up all correct and URLs and authentication settings double and triple checked. I was doing a lot of work on RS six months ago so it just had to be set up right.

Forgot I got a new machine a couple of months ago and never built the WS .dlls. -_-