The reference is to a bloody Boer ambush at dawn on 31 March 1900 at Sanna’s Post, Koorn Spruit Drift, that resulted in almost 600 British casualties, killed, wounded & captured.
See http://www.clash-of-steel.co.uk/pages/battle_details.php?bat... or http://en.wikipedia.org/wiki/Sanna's_Post for an out line of the battle.

Cpl Osborne was reported missing at the Sanna’s Post action in The Times, Saturday, Apr 07, 1900; pg. 12. Casualties: The Koorn Spruit Ambuscade, as one of those listed with missing of Robert’s Horse.

This was “corrected” in The Times, Friday, Jul 05, 1901; pg. 9. The War. Casualties; Corrections.

“Court of inquiry states that 2004 Cpl. G. J. Osborne, 14th Hussars, attached for duty to Robert’s Horse, reported missing, was killed in action, Sannah’s [sic] Post, March 31, 1900.”

As he died in the war his record of service will not be in the records of WO97 where the records of soldiers who lived to tell the tale would be.

The 14th [King’s] Hussars are now part of the lineage of the King’s Royal Hussars and their museum is at Winchester, the 14th [King’s] Hussars museum and archive is at Preston Lanc’s.