Before thinking about Continental 767's isnt it best to establish the 757 first.
A look at Continentals web site shows the web booked seats and these can be compared to the other UK flights.
The worrying thing is that the fares from Bristol seem to be considerably lower than elsewhere on the random dates that I checked and the also the front does not seem to be very full.
Take a look and decide for yourselves.
The viability of BRS and BFS also depends on how it affects the other established routes.
Centre cities