Steady-Eddie:On March 6th I was contacted by a member of npower's telesales team
Is the March 6th phone call date right because that is still within 14 days of March 19th?
Cooling-off periods vary from the minimum of 7 days to 14. If npower's is 14 days that's fairly generous. I think you were just within it on March 19th and were given factually incorrect information that the notice starts with the telephone call. There is no form of notice which starts at least until the next day and probably until the paper copy is sent with a deemed allowance for the post.
Haven't seen the npower contract so can't be certain about the cooling-off period but on the face of it you were JUST within the 14 days when you called.
You say you complained and the complaints team responded. What you MUST do is to obtain the npower Code of Practice on complaints and follow it scrupulously. There is a defined escalation process culminating in the right to complain externally. That stage costs nPower money but you nothing. You do not need to take no for an answer.