My D plate also has red light on the ABS belt. Get under the car if you can, the sensor may have fallen out. They only clip in & one of mine kept falling out so I eventually silicon'd it around the edges. B'cos both of them are the same (but one flipped over for the other side) one sensor fits on the top of the belt unit, the other on the bottom. It is the passenger side which is on the bottom & this was the one which kept falling out.

note: you MAY be able to see it hanging down, have a look behind the gearbox area on the passenger side. If you can see some wires with a small black box attached with a shiny metal clip on it, then this is the sensor.

If both of them are still in it could be one of your belts has snapped.