Iain, another good one which does a 'deeper' stretch is;
Lay on your back and bend your right knee (foot still on floor)
Then with your left leg, rest your left ankle / lower leg on right knee.
Grab hold of your right lower leg by putting your left arm through the 'gap' and your right arm round the outside of your right leg.
Bring your right knee to your chest and feel the stretch in your left hip.
Repeat on the other side.
Btw, I had to do this in the comfort of my office with my skirt up by my waist to see if I'm describing this right! lol! Good job I work alone!
I don't just concentrate on my hamstrings but also include the IT band, Psoas muscle and glutes to get a balanced stretching routine.
Hope this helps.