The Rail Accident Investigation Branch (RAIB) has released its report into a fire on a HGV shuttle in the Channel Tunnel on 21 August 2006.
The full report is available here:

A summary of the key points from the report are: On Monday 21 August 2006 a fire broke out in the load compartment of a lorry on an HGV shuttle train from the UK to France. The train was brought to a controlled stop 20.5km from the UK portal and all 34 people on board were evacuated without injury. The carrier wagon was structurally damaged, and there was some damage to the overhead line and the tunnel lining. Normal operation of services through the tunnel resumed the next afternoon.

The immediate cause of the accident was a fire in the load compartment of a lorry on the penultimate wagon of the HGV shuttle. A contributory factor was the difficulty in detecting a smouldering fire within the lorry's load compartment. The report also identified several factors relating to the effectiveness of detection and surveillance in the terminal, and the effectiveness of the incident management.