Hamish, your swing looks better than your handicap may suggest. Although I can only see your swing face on I suspect you may 'push' slice your drives rather than a traditional slice and shank because you slide and lean into the ball too much.
Excessive knee and hip slide causes your club to 'drop' and flatten behind you and the only way to get the club to bottom out is to sink with the knees.
Your aim should be to keep the club shaft hinging more up and down whilst turning and not shifting as much through the ball.
Practice with your feet close together and with a tee peg in the butt of the club - aim to keep the tee pointing toward the ball or even more towards your toes as you swing down.
This will keep your swing more on plane and the club descending on the ball along the correct path, briefly straight then arcing back around to your left.
Your legs will feel 'quieter' with the drill, try to take this feeling into your normal swing. Also practising off an uphill lie will help. Best of luck.
» You can submit a video of your swing in the Rate My Swing section of www.todaysgolfer.co.uk.
For more tips and instruction from Adrian Fryer visit www.adrianfryergolf.com.