Can you get hold of a foam roller (around £10). same effect as a tennis ball- but easier to use for larger areas. When rolling your calf over it very slowly- rest the other leg on top of the lower one to provide more pressure. When you come across an extra sore bit- stay in that position for 10 secs or so (if you can bear it).
Do this daily and pretty soon you will get rid of the knots - you will know because suddenly using the roller wont be painful anymore.
I use the roller everywhere at the first sign of a pain- and also weekly just to check all is ok (kind of preventative medicine). I find the tennis ball good for more specific areas - just above the knee/ hips etc.
Yes it hurts when there are knots but it is the way to solve it.
I see a running coach for weekly strenghtening exercises (after suffering many injuries!) - if sh finds a knot in my legs somewhere - she basically pushes down with her thumb really hard into my leg at the sore point to relieve the muscle!
Enjoy