I was just going through my notes of Sivyers who were in the army and one of them may refer to your gg grandfather Caleb.
Caleb Sivyer of Silver Hill, Hastings joined the Royal Sussex Reg 9th August 1898 at Brighton age 18yrs 2mths aand was discharged 4th May 1901 medically unfit. 5' 5 2/8" and 127lbs. If that is your gg grandfather he probably added a year or his age as he wasn't with Samuel and Maria in Hastings in the 1881 census.
Apologies if I am telling you what you already know.
You may also be interested in my posting of 13/09/2003 where I have gone back further with the James and Sarah Sivyer line.