Thank you Pavel.
I did try your approach and it works nicely for the Value but doesn't always update the displayed sign when the updateValueEvent is in Lost focus mode.
I am using the following and seems to do what I need it to do:
//if + key is detected remove the negative sign
if (e.Key == Key.Add || ((e.Key==Key.Unknown)&&(e.PlatformKeyCode == 187)))
// this is the control