I don't think this William is the brother of the Howard buried at St Barnabas. See # 63This is spooky as I was just looking this chap up not 30 mins ago! There is a listing for Wamble which is miss spelt it should be as you quite rightly say William. South African born in Trans-val.
Buried at Helles Cemetery Turkey. Appears to have a brother Howard. Don`t know of another brother but these two are on the memorial at St.Barnabas. Howard is in the RN and therefore his body may be lost. Or buried at sea but more then likely lost. Hope this helps?