There is an alternative course of action prior to taking this to court (which should always be a last resort). As you have already tried their complaints procedure, and they have failed to sort it, you should contact Consumer Focus, who act on behalf of Ofgem (the industry regulator). They will take your case to NPower. They have more clout as they can log complaints against NPower.

Also, good luck with trying to charge them for phoning you. In 4 years of working within the complaints department of a power company, i never heard of one person getting paid for receiving phone calls.