Thanks, Cathus and OldBoy.
Unfortunately my flashgun isn't that sophisticated; as far as I know it does not have high speed settings, and there is definitely no dial to adjust power.
I need to upgrade
But there is so much kit I want.... better glass, better body...
And far too much discrepancy between the cost of the kit and the coloured paper in the wallet. That seems to be a common difficulty among photographers
Thanks again for the advice.
Les