My memory must be failing as I'd completely forgotten that we went over similar ground last August
http://www.rootschat.com/forum/index.php/topic,173513.msg825891.html#msg825891It's very possible that the Lt Gransden Ameys moved to Cambridge, but you need to check Lt Gransden parish register to see if the Ameys come to an abrupt stop around 1797. If they do and there's no burial in Lt Gransden that would explain this then that increases the likelihood that the Cambridge Robert and Mary are the same couple. But you'd need to check the Cambridge registers to make sure that there are no Robert and Mary entries concurrent with the Lt Gransden baptisms, which would indicate there were two couples (I doubt it!)
As I suggested before, looking laterally at the other sisters baptised in Lt Gransden might provide the link you are seeking. Did any of them marry in Cambridge and were alive in 1851 showing a birthplace of Lt Gransden? Check the Cambs FHS site at
http://www.cfhs.org.uk/Search.htmlwhere there are various indexes which might give clues - eg the burial index shows Sarah Amey aged 4 buried in 1801, which fits neatly with the baptism of Sarah in Lt Gransden on 25 April 1797, but you'd need to check the St Andrew the Great parish register to see what the additional information is.
The IGI shows a baptism of a Sarah Wallis Amey in 1810 at St Andrew the Great, daughter of Nancy Amey. More circumstantial evidence (a Nancy Amey dau of Robert & Mary was baptised in Lt Gransden in 1793).
Amey was not a common name in Cambs around that time, so it's not too onerous a task to pull out all Amey entries from the various (free!) online sites and try to connect them all up
Microfilms of parish registers can be ordered from and viewed at your nearest Family History Centre of the LDS - a list is on
www.familysearch.orghttp://www.roots-boots.net/ft/amey.txt might also be of interest
Regards
David