The math department is within walking distance from the train station (~30 minutes).

The map on the right suggests a path. It migth not be the absolute fastest, but you will get a nice view of the cathedral and the castle.  The precise address of the math department is:

Department of Mathematical Sciences
Lower Mountjoy Stockton Road
Durham, DH1 3LE
United Kingdom

All talks will take place in CM301, on the second floor (or third floor depending how you count?) .