I can't help with the chef questions but would just like to say, don't miss out on your opportunity, they don't come along that often.
As far as an agent is concerned, there are many threads on here about this and it comes down to your personal choice and situation (obviously!) I used an agent because my application was quite complicated, we wanted to ensure it was all done properly and I was also told they get priority processing with state sponsored visas. This now I believe is not true although in our case the visa was granted very quickly.
Good luck with what ever you decide but dont take too long!!