Is there a way of seeing what your take off time was whilst in flight (if you don't have a route planned).  I guess you can see it on the PLOG if you have a route however most of my flying is taking people up for experience flights and so I rarely have a route in but I do need to know how long I've been airborne and am frequently forgetting my take off time!

You can switch to the PLOG during flight by tapping the position report at the top of the screen. If you actually have a flight being logged, you should see an extra section at the top detailing your takeoff time and how long you've been flying, etc.