I got some more work done on the rack, I did decide to put a third strut on the rack, and I got the pannier rails done too.....

This will allow me to use the panniers even with the trailer in tow, I won't be able to use the top of the rack but I think that is OK.
I will make up some sort of attachment, a stud for lack of a better word, that will mount on top of the rack to attach the trailer to, it will become the hitch I guess.

Just playing with ideas here.
One thought I had was to make something that hooks around the cross bars on the rack, and then is held in place with one bolt big knob. This would allow me to easily remove the hitch from the rack if I wish, but would also be a very solid mount for the hitch.
More on that tomorrow, it's 3AM and I need to get some sleep!
Thanks to everyone who has commented on this build, it is really great to hear all of the various ideas, it makes me think
Cheers!