This is amazing how all this is fitting together.
The diary was written in an almanack or similar published in 1756.
http://en.wikipedia.org/wiki/Rider's_British_Merlinhttp://freepages.history.rootsweb.ancestry.com/~simba/ridersalmanac/homepgalmanac.htmthe latter link mentions that the 1758 edition they have contained the accounts of a Warwickshire bailiff. So the book must have been published with a lot of blank pages intended for keeping notes or a diary.
The note on Lester street and swallow street is right at the beginning, the reverse of the pattern inside the front cover can be seen on the left page.
It reads
"Mr John Miller at Mr Dunn's shoemaker opposite the five bells in Lester street in Swallow Street. French cheeses Chesnuts (sic) French pears".
Then a line drawn across.
"Marcket (sic) in Church Street fronting St Ann's church an (or on) Balshop (?). French and Limbourg cheeses"
So I think written by an adult lady keen on French cheese etc. It looks like Mr Miller may have set up a pop up shop at the shoemakers to sell some food items he managed to get from France. So presumably written at a time we weren't at war with France, which does sound somewhat earlier than 1803.
It looks the diary was started by an unknown lady living in London and taken over 20 or 30 years later by a 12 year old girl in rural Sussex. I am not aware of anyone in the generation before Sarah coming from London or spending time there, so a mystery how the book came into her possession.