As for both on the same isp thats just the way the cookie crumbles. Inevitably it will happen as companies now days are always doing deals for new customers, and some also have retention deals.
The £13.75 is probably not a sustainable price, ie. if everyone paid that price service would suffer. Normal price seems to be just over £20 for what you have.
BT Infinity 2 Since Dec 2012 - Estimate 65.9/20 - Attainable peak 110/36 - Current Sync 71/20