KGarrard is correct - the 14 days is the duration of the illness.
Do you think that there would be a copy of a medical cert. out there somewhere in someone's records: highly unlikely, but a fascinating thought. M
The medical cert (called the MCCD - Medical Certificate of Cause of Death) is retained by the registrar, but most of the information on it is repeated on the register entry so wouldn't help with any research.
Most doctors don't fill in the duration of illness column any more, and even if they do it is no longer included on the register entry.