Thank you both for your answers and pointers in new directions. I was completely unaware of the National Archive holding military records or that Spencer's records may still be available. I have also not searched the London Gazette previously, so can use that source in future. Phil, would the sentence 'with seniority 28 Nov. 1916' point to Spencer having been with the Reserve Cavalry since that date? or having been a Temp. 2nd Lt. since that date?
I've located a copy of the Hertfordshire Yeomanry by J.D. Sainsbury which will give me details on their movements during WW1.
Thank you again for your help - and I'm amazed once more by the knowledge of people on this forum.
Sara