Bernard Trevethan was born in Leamington, Warwickshire on 15 October 1920, the younger of the two sons of Edward Trevethan (from Cawsand in Cornwall) and Fanny Trevethan (née Vercoe) – his brother was Edward and he also had four older sisters, Queenie, Doris, Violet and Phylis. In the 1921 Census the family were recorded as living at 14, High Street Leamington and his father was noted as working in the civil engineering department of the London & North-Western Railway. The family later moved to Stoughton in Surrey and Bernard was educated at Stoughton School.

After leaving school Bernard Trevethan joined the Royal Navy on a twelve year Continuous Service engagement. On 4 May 1942 he rescued a woman from the railway tracks at a station in Surrey as reported in the Surrey Advertiser and County Times of 16 May 1942.

Bernard Trevethan was a volunteer for ‘Special Service’ and he was drafted HMS VARBEL and trained as a charioteer. He was paired up with Sub Lieutenant George Goss as his No. 1 Crewman and was his No. 2 Crewman. A night training attack on the battleship HMS HOWE was successful and they were able to place their charge under the stern and return undetected to the Depot Ship.

Bernard Trevethan was selected for a team of charioteers formed for an operation to attack Italian shipping at the port of La Maddelena. The team was embarked in HMS P311 for the operation with three chariots – Nos. X, XVIII & one other. P311 had completed the passage through the Sicilian Channel – reported at 0130 on 31 December 1942 – but no further reports were received from the submarine and P311 was presumed lost with all hands in a minefield near La Maddelena on or about 2 January 1943. Also lost were the three chariots, the three chariot crews and the team of four dressers – ten personnel in all. The date of the loss of P311, her crew and her chariot passengers was assumed to have been 8 January 1943 which is the date that the submarine was due to arrive back at Malta.

Bernard Trevethan listed his parents as his Next of Kin – then living at 36, Rydes Avenue, Stoughton in Surrey. He is commemorated on the Portsmouth Naval Memorial on Panel 75 Column 3 and on the 12th Submarine Flotilla memorials at Rothesay on the Isle of Bute and at Kylesku.