I've just received the marriage record for JHT Parr and Amy Newman. The name given on the record is Jack Henry John Parr, the father's name and occupation being Thomas William Parr, horse keeper. His age matches his birth record to within a year so I'm pretty certain it's my father's brother.
I've also found three Electoral Register entries in Lambeth for 1929, 1930, & 1932, in which Amy and Jack are listed.
But there's still no trace of him (that I can find) in the 1911 census.
There is a death record for a Jack HJ and a Jack HT (both point to the same record) in Holborn 1960 and I'm therefore inclined to accept this as the same person. I probably need to order the death certificate but I'm not expecting it to tell me a lot.
I've not been able to trace probate records for either Amy (died 1973) or Jack (died 1960).
So for the moment I'm looking for other children of JHT & AA, and anything further on Olive ...