Cheers all. Yeah I've been slapping for years and thought I've got the wrist 'roll' down, no movement past the elbow.
I've recently increased the amount though, maybe just doing too much too soon, possibly not relaxing enough (ironic as like many of us I play to unwind after a day at work).
JTUK completely agree on height - obviously practice a lot sat down, and when standing, the bass sits about 2inches lower.