I stopped immediately after Bray Boats (Ray Mead Road) moorings last year (august) for shopping and someone did come to charge £8 or something but i was not on board (just an innocent casual observer on the pavement!) and had untied and cast off before he came back. I think it was Windsor and Maidenhead Parks department or similar.
I heard that if you go up past the islands and stop by the road there (the same side - left hand bank going upstream) before the lock cut bollards it is a different land owner and not chargeable. Not sure about access steps though for that bit.
It did appear to be true as there were several boats there when I passed on way down River a couple of weeks later.