I DIYed my older 2 DC and both passed for their first choice grammar. I had exactly the same doubts as you though, and at around the same time. It was first time round for me, all my DC's friends had private tutors and I wasn't sure whether I was missing out something vital in my DC's preparation. As it turned out my DC got places where some of the tutored DC didn't, so there is no magic involved with a tutor.
There is so much really valuable info on this site. Make sure you read up on what the Birmingham exam may contain because it can't all be covered by the standard Bond(etc) books. I made up a lot of my own resources, particularly for vocab and cloze. I also found the Manchester Grammar papers really useful for practice closer to the exam.
More than anything though, the advantage you have if you DIY is that you know your DC and their strengths and weaknesses. This is something it could take a tutor valuable weeks to work out.
You will have lots of wobbles between now and September I'm sure, but if your instinct is telling you to DIY carry on and don't worry about what others are doing.