So, I managed to find a proper "Hello World" program for my PDP-11 on the internet. It can be typed into ODT without too much effort. This is what happened when I typed it into my machine:
As you can see, I'm using this to disable interrupts and start it:
It's a useful little test. I now know that my machine is capable of running programs, which is good.
So my next step is to try an RT11 boot disk so that I can attempt to run an operating system. The ultimate goal is to run Unix rather than RT11, but you've got to start somewhere...