i had an interview with b&q 2 years ago amd im sure everything is pretty same!
we had a group interview and i think they do that when they have more candidates than 5-6 (this is only my opinion).
just like any other group interview you will do some tasks and interviewer will watch you!
and if they think you did well then you will also have a one to one interview.
they will ask you typical interview questions and let known about salary, hours etc.