Well, what can I say other than thank you - once again the value of local knowledge (including that of how the records work) wins the day. that information gave me three generations of the family and takes me back to the mid nineteenth century.
Just to make things more interesting, however, here's another bit to add to the mystery which I have only just found:
Ireland's Memorial Records lists
PENTLAND, WILLIAM. Reg. No. 9080. Rank, Rifleman, Royal Irish Rifles, 1st Batt. ; died of wounds, home, August 26, 1916 ; born Ballymacarrett, Belfast.
whose servcie number connects him to
Rifleman Service No: 9080 Date of Death: 26/08/1916 Regiment/Service: Royal Irish Rifles Grave Reference B. 638. Cemetery CANTERBURY CEMETERY, KENT
It would be hard to believe that he is not a brother of Samuel but the censuses don't suggest it.
I tried checking the Ulster Covenant but there appear to be no male Pentlands (although there are two females) in East Belfast, i also tried wild card searches but it seems the search engine isn't up to it.