I recently complained to BT about the large increase in my monthly bill. I'd sent a very grumpy email! I was contacted by a very nice Irish lady who talked me through my complaint, posted me a hard copy of my bill having explained it, and transferred me to another department.
Long phone call. However, end result was massive series of upgrades on phone, broadband, TV, netflix etc and all for almost £15 pcm less than I was paying before. Yes it was a palaver and took time but the end result was extremely positive. It certainly paid me to complain. Go on Grans - make a fuss, it could be to your advantage!