So the roadworks are finished, unfortunately a bit of bad luck the larger tunnel is on the right hand side serving lanes 3 & 4... see page below for more information, and useful video, especially relevant for drivers of larger vehicles

http://www.highways.gov.uk/roads/road-projects/dartford-crossing-remote-payment-scheme/

When the bridge is closed, and the tunnel taking all traffic, the large vehicles over 4.8m high are banned northbound as the northbound just uses the smaller tunnel - under normal conditions larger vehicles use lanes 3/4 northbound but can use lanes 1/2/3 southbound.