Ureka! I've got it.
I received this e mail response from Satmap support,
Main menu>settings>compass settings>grid magnetic angle>”automatic”
Main menu>settings>GPS settings>use GPS compass>”1kph/0.6mph/0.54”
Recalibrate the compass - Main menu>settings>calibrate compass>
I had done the first and last correctly, however in GPS compass settings, I had set it to 3 mph instead of 0.6 mph, as I thought it was to be set to normal average walking pace.
Just set a route to test around the block, and it's bang on now. No fluctuating, steady as a rock.
Interestingly too the blue circle direction pointer has suddenly appeared.
I now think I'm in love with my Satmap.
Thanks for all your help, Les and SC.