I'm new here and an utter car ignoramous to boot, but I'll explain the problem as best I can.
It first happened when the snow got bad - I was coming down a hill, tried to slow down but just slid. I was pressing the brakes hard but was just getting a loud grinding sound and a scraping/juddering which I could feel through the pedal. I tried pumping the breaks, which helped to slow me down, but if I held the brake pedal down, the grinding returned and I kept moving. I eventually steered into the earth bank at the side of the road to stop.
This morning, I had to beak hard because a moron in a lorry seemed to think he needed the entire road. However, the same thing happened as before - the juddering/grinding, and me still moving.
If I brake gently, everything seems fine, but if I brake hard, I slow down a little, then the gringing/juddering kicks in and I just seems to roll. It feels like the wheels turning but the brakes just aren't catching - that's what it feels like, but not being able to see, I could be wrong. It cerainly doesn't sound like I'm skidding, which is why I presume the wheels are still going around.
Sorry for the simplistic explanation, but I do hope someone can tell me if I have a problem or not! Oh, and it's a 3 yr old Sirion that had a service in October, if that helps.