Thanks again all and to MedPat, I have now obtained a further 2 death certs of the same family members, one who died 13/2/1859 aged 2 yrs and one who died 17/2/1959 aged 9 months: both causes were Croupal Diphtheria: one stating 7 days the other 21 days. So you are correct.
RootsChat is such an excellent site with many many helpful and knowledgeable members.