Thanks for the feedback folks, much appreciated!
Gadget, useful links but like you I have not found anything that fits in with my suspicions. I will follow your advice and expand out the search.
Giblet, good link on the epidemics opens up a number of possibilities of what the cause or reason could be. This timeline link is a good one for anyone searching causes of sudden deaths.
According to the death certificates/parish records my ggg grandfather died in 1840 of "liver condition" and buried the same day he died and my ggg grandmother died in 1844 of "cramps" and buried the same day as she died. So, neither of the certificates mention any specific diseases but is it likely that the true cause will have been left off purposely? I'm not a medical person but can you die of cramps?
Gwynd, in answer to your Cilmelyn question please see the new post (Cilmelyn Farm, Bangor?) on the Caernarvonshire board. This will save any confusion on this post.
Thanks again much appreciated

100%Gog