£400 seems to be a bit steep for a couple of rods and some earth cable and crimps. But then again it will have to be tested to ensure it is ok and someone will be supplying a certificate with their name on it saying that it is fit for purpose. If you are going to run in a new supply to your shed you first need to work out the amount of current you will need, then the length of cable run. From this the cable size can be calculated. If you need help with this post the figures and I will work out the cable size for you.

Regards

Ian