Cyril was born on 25 April 1899, the sixth child of John and Catherine Teresa (née Connor) Jeffery and baptised on 7 May 1899 in the Cathedral of St. John, Portsmouth. Eventually the family unit consisted of Mum, Dad, 3 brothers and 3 sisters.

On 19 May 1915 he joined the Royal Navy when he walked over the gangway of HMS IMPREGNABLE the “Boy Training Ship” based in Devonport as a Boy Second Class to start six months basic training, following in the footsteps of his elder brother John that had 13 years before joined the RN  in 1902. On the 17 July he was made Boy First Class and on the 27 November was drafted to HMS VICTORY I staying for 3 months. Then a draft to HMS MAIDSTONE on 5 February 1916 introduced him to the submarine service. On 22 April 1916 he joined HMS BONAVENTURE still as Boy First Class but on 9 March 1917 he was drafted to HMS DOLPHIN as an Ordinary Telegraphist and on 16 April he was drafted back to MAIDSTONE, based in Harwich, for HMS E47, part of the 9th Flotilla.

E47 was reported lost on the 20 August 1917 with no survivors from her 32 man crew.

In 2002 a Dutch Diving team pinpointed the wreck site and exploratory dives shows what appears to be mine damage causing large hole on the Port side around the control room area. The bow lies in an Easterly direction. The deck gun had been ripped away from the deck by fishing net activity and this was salvaged and the serial number identifies the wreck as that of E47.