The NCS will make sure that your form and all the documents are in order, and will return your documents to you (they send certified copies with your application).

I thought that the convenience of having my passport back would well be worth the 47£ fee for the service, but it's more than that.

My application would not have gone through because I fail residence requirements (this was unexpected: being married to a Brit isn't sufficient as I have to exercise my treaty rights as well). Not only did this save me the 735£ Home office fee, but they didn't even take my money as the application wasn't submitted.

So use the NCS!