I had another go last night, and whilst not getting a super O, there were some interesting sensations.
I set myself up on my bed, with a laptop on the bedside table running a random slideshow of erotic pictures.
I lay there with both knees together and bent.
After about half an hour I started getting some nice feelings which I could influence by thrusting forward and contracting or backwards and bearing down; not involuntary contractions as described but I have the feeling that they might have lead on to something higher if I had a bit more time. I seemed to be getting to the stage where I was trying to contract all the time and tensing my body quite a bit.
Well, it's encouraging, hopefully I'm getting there, and I cannot wait for the chance to have another session.