I know what you mean about relaxing the grip Gaz I don't think it was that cause I have practiced that quite a bit ???
It might be because I didn't have stomp grips on the tank due to the paint only just being dry, I'll see the next time I'm out cause I've just fitted them.
A suspension check up is defo on the cards though 
For all the practice, you have a lot to think about riding on track so it wouldn't be unfeasible that instinct takes over once in a while.
According to a course I've recently attended, a bike will correct even the worst wobble by itself within about 0.5 of a second, due to gyroscopic properties of the wheels; a tense arm will attempt to stop a wobble within 0.012s, which causes the wobble to increase to the point of a tankslapper. This was proven by riding over (small) logs at 20-30mph with hands completely off the bars* - no falls.
On my ZX6R, at Clay Hill on various laps I could feel a noticeable twitch in the bars as I went over the lip. This was on full throttle in 4th or 5th. When I consciously tried putting zero weight through my arms I couldn't even feel them move at all.
At the little dip between Druids and Lodge, I had the start of a tankslapper in the CBR. Near enough took my hands off the bars and the bike calmed down in less than a second. I think I may still have got passed, but I was more worried about stopping the poop draining into my boots at the time.

I personally think that at our level and the bikes we ride, a steering damper is not required on a 600. Buy another trackday to get more practice

*Except me, cos I was too chicken to manage it.