You were spot on about Dorothea being a Reitz! Interestingly on the census data she is always listed as being born in Germany, however there is a birth record for her stating she was born in the Plaistow area.
I did know about John Diebel but haven't got around to researching him yet.
The fact that there is no Diebel surname in the Leipzig directories for those years is also interesting.
Thanks for your help :)