These viewing figures are calculated by some boxes ontop of about 5000 people's t.v's up and down the country, so extrapolating the results of 5000 people's viewing habbits to 26,000,000 t.v's is about 0.01% of people watching. It's like doing a survey for London and asking 5 people down Camden market what they think. I think Channel 5 will be weary but they will also focus on how much money they make from BB to base it's success, because higher ratings does'nt always mean more money.
I think load's more people watch big brother thank people think, but they won't be counted unless they have a BARB box, so I'd take these figures with a pinch of salt, The fact that C5 did'nt do launch advert's wont have helped though.