The Time came..
All done and dusted, easy-peasy.
I note that a full daily scheduled scan takes 2-1/4 hours for 660K items, including my two external HD's, but since it doesn't appear to slow anything down whilst it is running I'm not bothered.
Strangely enough I noticed one side effect which relects on Green Granite's post about program hanging on shutdown - previously when shutting down I would get past 'saving your settings' when a splash screen would pop up for SuperAntiSpyware - 'ending program' and the blue blocks scrolling all the way across and pausing until 'this program is not responding - End Now or cancel' An End Now would terminate it and the shut-down would complete.
Since removing McAfee and installing MSE, on shutdown the same splash screen pops up and after scrolling three blocks it disappears of its own accord and PC shuts down.
Thanks to all for the advice and comments.
DD