Go down to bottom of forum page, read 'joining the royal marines reserve', its got some good accounts on there, pitty he had to stop though!
Just dont get phased out on your selection weekend, I think thats why most people fail, thinking they are not good enough. If you are not good enough the instructors will soon tell you! Remember you dont have to be superman to pass it, they want to join at the end of the day.
Weekend exercises? Prepaired to be wet, cold, hungry, sleep deprived...