There is a very informative service record to be found on Ancestry / Fold 3 in the database for UK, Royal Hospital Chelsea Pensioner Soldier Service Records, 1760-1920
for a Matthew Leigh born 1879, Northwich who originally attested in the South Lancs (no 2 5313 - altered with thanks to AndyJ2022) on 10 Jun 1897.
He was transferred to the Grenadier Guards on 19 Aug 1897 which I assume, but I'm not brill with military records, is when his original service no was crossed out on the record and 6786 written in.
To help you/the gentleman who is enquiring to decide if its the right person the record says:
Father: George. Mother: Ann Jane, older brothers: Harry, Thomas, George, younger brothers: Frank, Fredk,Arthur, Harold, sister: Sarah Jane
all of 5 John St Northwich
Boo