In reply to Andrew Mallinson:
Get yourself a solid rubber ball, about the size of a cricket ball from a pet shop.
You're expecting a punchline, aren't you?
While sat on the ground, or with your leg resting horizontally, place the solid rubber ball under your calf and rest your leg on it. Try to move your calf around till the ball rests against the most painful, or tight knot that you can find.
The solid rubber ball will then act like a physo's hand/thumb and attempt to break down any tightness or knots.
It just so happens that I am doing this exercise right now.