The"Muck Hole" as it is known is to the north end of the station. If you have arrived by train then leave the station and cross over the main road at the front of the station building. Keep slightly left you will cross over the overbridge for the freight lines, in front of you you will see the station car park and BTP building. Walk into the car park keeping the fence alongside the railway cutting to your right, follow fence to bottom above tunnel mouth. Keep right and follow path to footbridge, from top of footbridge you can see all traffic on freight lines and passing through station (binoculars may be needed). Good vantage point, no facilities or shelter from the elements, photography of muckhole traffic is somewhat limited without a telephoto lens.
Average number of workings per hour via muckhole is 4 to 5, made up of light engine movements, engineers trains, intermodals, etc. There is also access back on to the station from the footbridge though I can't remember which platform it takes you onto.
Happy spotting; regards; Sammy