Although Holy Trinity is a garrison church it has a White Ensign (the flag of the Royal Navy) above the pulpit. It was laid up in memory of naval personnel connected with the parish on Sea Sunday, 8 July, 1979.

For many at Holy Trinity it is associated with Earl Mountbatten of Burma, a Royal Navy officer and Admiral of the Fleet.  It has also been suggested that it came from his ship, HMS Kelly (from the records at the writer’s disposal this seems unlikely as its installation was a separate event from Lord Mountbatten's visit).

Earl Mountbatten attended a service for the Life Guards on the morning of Sunday 22 July 1979 - he was Colonel of the regiment. Sadly this was his last official public appearance before his assassination the following month.

In the summer of 1979 building work was underway to create an additional room on the south side of the church - the Choir Vestry had already been added in the 1930s.  It was decided to call the new space the Mountbatten Room.  It was dedicated by the Bishop of Buckingham and opened by Lady Joanna Knatchbull (granddaughter of Lord Mountbatten) at a special service on Trafalgar Day (21 October) in 1980.