Hi Katy,
Have you checked your usage for the last year. 178 per month sounds high, though it is a 4 bedroom, so expecting to pay more on top of that sounds extortionate.
I would consider writing a complaint to NPower about this as clearly that monthly amount is not a low usage cost, nor is the fact that they didn't highlight the incorrect tariff to you at any point.
A comparison on this site using the annual usage and the other details should indicate the type of tariff that you should be on.
HTH
Sparky.