You write "Thorkild Jacobsen dates the Eridu Genesis to 1600BC, some say 1800BC, but I have read commentary that it comes from an earlier original c. 2300BC. Killin me I can't find the original source though it is blindly repeated on the net."
If you look on the ePSD (electronic Pennsylvania Sumerian Dictionary) - in other words, if you prefer to go to the best source over and above yours truly - and type in ERIDU, it will come up with a symbol we already discussed together: NUN that ressembles Egyptian djed. ERIDU can be further dissected into ERI/URU and DU, city and foot/establish, and might have the meaning of establishing a city. You will find NUN/ERIDU used in this context at the beginning of the so-called Sumerian King List, lines 2 and 3, ETCSL transliteration reference 2.1.1.
My second point requires the obvious leap of faith. The oldest flood story in terms of Sumerian literature is most probably mine; the re-translation of The Instructions of Shuruppak; For the age of the oldest tablets bearing the text, you can check on Wikipedia. It's around 2600 BCE.