  A programme of detailed simulations of the response of the Durham Mark 6
atmospheric Cherenkov telescope is in progress. The effective collecting area
for triggering by gamma-ray showers after application of selection criteria is
derived as a function of energy. An initial result from the larger events in
the 1996 and 1997 observations of the BL Lac PKS 2155-304 is that the time
averaged flux above 1.5 TeV was (6.7 +/- 2.2) x 10^-8 m^-2 s^-1.
