We filled out the form 4 years in a row. The only time you hear something back is when they want you to take part. I have heard they are not doing anymore shows.
I emailed them a few months ago to find out if there were going to be any more shows and got a automated response of 'here is an application form' (not what I was looking for, but anyway...) and a 'we don't know if we are going to do any more shows, but if we do, heres the application form'.
I then got another email a few weeks ago saying 'we are going to do another series so if you want to apply, make sure you return that application form.
I would imagine its like most other tv programmes and if you don't get a response unfortunately you are not what they are looking for, however most tv seems to be made very last minute, so if you haven't heard anything yet it doesn't mean they don't want you maybe.
Hope that helps, good luck :)