When you say H G Field, if you are actually talking about Horatio George Field, then I just looked at the relevant rolls namely, WO100/268, WO100/282, WO100/354 and then compared those with the relevant entries in WO127, the originals of which are at Kew.
I suggest that he was attached/seconded to the Scottish Horse and certainly not "recruited" in the actual sense of that word.
I think that when researching these men, it is always best to start at the very beginning, I did see a very likely match in RG10 regarding his birth.
Court wrote: Thanks for the information. May I ask how it is you know he was a subaltern in Kimberley and had endured the siege in Kimberley?
I only ask as it was my understanding that he was in the Mafikeng siege and the Mafikeng siege notes we have are testimony to this. Having said this I have found some association for HG Field with the Cape Police - but no historical record tying him to that unit, either before or during the war.
How would I go about tracking his activity before the war? I find it odd that he would have been recruited to the Scottish Horse as a Captain, and can only assumed he had a military history prior to recruitment.