What went so wrong at Tesco?

Tesco's grim Christmas results - the worst in many years - has had a crushing effect on its share price: this morning Tesco shares plunged 15%. That's £4bn off its share price. But it's not for want of trying: Tesco launched an aggressive £500m "Big Price Drop" campaign last year to attract consumers under pressure from surging food and fuel inflation, job worries and energy power hike rises.

What, then, has gone so horribly wrong for Britain's biggest supermarket player?