I think the show is filmed in the autumn and only the finals are still to be decided.
I think the show has become more mainstream since the first series, and this increase in popularity, at a time when a lot of other reality shows are seeing a decline, has probably changed the type of people who apply and the most successful candidates. From the first series Miriam stood out as a really good contestant, but if she were in it now, most of her contributions would be left on the cutting room floor. "Entertaining" characters likely Tre and Katie dominated despite it being obvious that they were not good enough/wanted to work for Sir Alan.
I think there has been a big decline in the footage of the tasks in each episode. By the time the what happened last week/Sir Alan arriving/task intro has been completed; they show 20mins of task; and the rest of the show is the politics/boardroom/firing. I think it has become more like a reality show - but it is still a great format and is a better alternative to other shows.