The certificate finally arrived - five working days after being despatched!
Elizabeth Kennedy was living with her daughter, Ellen Partridge, when she died. Ellen was the informant. This means there's no doubt that I've identified the correct Elizabeth Kennedy. The certificate provides further confirmation in that her 'Occupation' is given as 'Widow of James Kennedy, Shopkeeper, Confectioner'.
Thanks everyone for the advice given above. However, I have another little problem! James Kennedy features in the Ancestry trees of three families. Each of them identifies his father as William Kennedy, born 1825, and his mother as Margaret Kennedy, born 1827. Previously, I'd obtained the marriage certificate of James and Elizabeth, and James's father is named as Matthew Kennedy - not William. I assume the marriage certificate is correct, which means each of the Ancestry tree owners is incorrect in identifying William Kennedy. The tree owners are using an 1871 census return showing a family called Kennedy living in Newport; this surely must be the wrong family, even though the age of James in the 1871 census return matches his age from other sources.
I've just ordered another death certificate, that of Matthew Kennedy, died in Chepstow age 75 in 1898. It looks like it could be James Kennedy's father. Any thoughts on all of this would be gratefully received.
Thanks,
Chris.