Do you have a copy of the death certificate? Ref: Vol 3a page 217. This is always important to obtain.
You have a notice given to you, but it is a case of elimination of where the burial may be. Eton covers a range of the Bucks area, so obtaining the certificate may help a bit further. It may just be a process of elimination so you could google the church mentioned and then write to them. It cannot do any harm.