Ken
Just now I had a look at my useful Imperial Yeomanry CD and there are a number of entries under that surname. I can post a list (and their numbers and respective companies) and it might be possible to correlate one of them (i.e., coming from London Fields, East London) with IY enlistment papers. I understand that IY enlistment papers do exist and can be accessed on the commercial sites. David is much better placed to speak on that subject than I. You never know, you might get lucky! And I have had longer shots come off!.
Good luck and let me know if you want the IY BALL surname chaps listed.
Regards
IL.