I hope you haven't changed yet, I think I can help you with this one.

My Mum is on the Staywarm Tariff and I've been paying £74.64 per month for her.  She got the same letter, I started rubbing my hands with glee ....... 15% decrease on it's way.........then shock horror, a letter arrived yesterday raising the monthly amount to £90.57 per month from 1 June.  Her bill will be slightly less that yours because she is 1 person in a 3 bedroomed house.

I got straight on the phone last night 0800 404 6277 and asked what was going on.  I spoke to the really lovely lady who did a bit a quizzing.  She asked eventually, was Mum receiving any benefits?  I said yes, Pension Credit etc.  I had to give her Mums national insurance number and pension office address and she asked could I hold on a short while.  2 or 3 minutes later, she came back on line and said Powergen had been allowed to raise the Staywarm tariff prices for everyone EXCEPT those receiving benefits like pension credit etc.  Therefore they had reassessed Mum's monthly payment down to £65.89. 

Incidentally, her useage is massive - 4372 kw Electricity and 31829 kw for Gas!

 I hope this helps

Regards

Christine