Thanks for the responses.
I find that when I get the boat trimmed nicely and the throttle set to an appropriate speed, continually holding the top of the throttle lever results in me gradually, or all at one go, reducing the revs. So, then I throttle up again and the same thing often happens, sooner or later.
I hav tried kind of 'hovering' my hand above the throttle; that is to say, a very loose grip. This technique works but is a bit stupid.