Hi Deni,
I'm in exactly the same situation, just starting to look at the application forms. After reading through, although it looked really daunting at the time, once you make a list of what info you need to send etc I don't see what else an agency can offer if its a fairly straight forward application. Its expensive enough without adding on anything additional! I'm currently digging out all out doucments, old and new. (car insurance, mortgage statements bills etc to show how long we've lived together. We've lived together 10 years and married for 3 so hoping it won't be a problem. Good Luck