It invades our homes, dropping onto our doormats in its millions and costs the UK taxpayer a fortune to get rid of. It might be a menace in the mailbox but without junk mail, would postal services survive? Panorama reporter Tom Heap asks if unsolicited material can actually benefit the customer, whether junk mail is only good for one thing, burning it to heat his home, if the and investigates whether Royal Mail is addicted to the darker side of the letters business - scam mail.