Wex, as I noted earlier, nothing turns up for David S[tanley] Jones with the number 15650. As far as I am aware the records retained by the Grenadier Guards have
all been transferred to TNA and have been indexed although not necessarily digitised. However nothing comes up on Discovery for any of the permutations of David / Stanley / Jones or the number 15650 in WO437 (the Gren Gds muster books). The actual service records of former Guardsmen form part of the large tranche of documents sent to TNA as part of the MOD transfer, and have not yet been accessioned.
I'm not entirely sure where the OP found that number for his great great grandfather so it's hard to advise on where else to look. It may be easier to get his WW2 record with the Queen's Own West Surreys, and work back from there.