Thanks everyone for your input. Just to clarify things.
The baby was born and adopted in London to an English couple in 1954.
The birth was registered with the mother's name as the surname, - and as the child had an unusual middle name, finding the birth cert on ancestry was simple. And yes, it just had the page number and folio number which is why the original question about information on it if she was to order it. We don't of course know if the adoptive parents kept the same christian names or not after adoption.
The site where I found the adoption was on Adoption Database Quickbase which covers all countries.
I got a 'hit' immediately putting in information given to me by this child's sister who knew the name, (which was as unusual middle name) date of birth, name of the birth parents and location. Both birth parents have immigrated to two different countries but still keep in touch.
Thanks once again for reading this.