First of all, my sympathies. It's very hard to miss by one mark.
I'm sure Forumadmin will have pointed you to the Q&As (link at the top of this page), and perhaps to some of the threads specific to Slough.
I believe there's a joint appeal panel for some of the Slough grammars, including Herschel, but their arrangements lack clarity. However, the first step in the process must be (1) to demonstrate that your daughter is of very high academic ability and ought to have qualified at the 11+ [for the purposes of an appeal, it's better to be bright than hardworking!], and (2), if applicable, that she underperformed because of convincing extenuating circumstances. There is advice on both these things in the Q&As. You might also like to adapt the "Letter of appeal for Mary":
The second step in the process, if your daughter gets through the first and Herschel is oversubscribed, is stating the reasons why she needs to attend Herschel in particular. There is general advice on this in section C of the Q&As, and there are other posters who can tell you more about Herschel than I can.
If you have any specific questions regarding appeals, I'll do my best to answer, but I only give advice via this forum to parents who want to "DIY". There are consultancies that provide the sort of service you mention (with appropriate fees
), but I couldn't recommend any. If you have the time, you might be better off preparing your own case.