William and Eva had 4 children all born in the Marylebone area. They had 3 girls and 1 boy. I'm not posting their names as they are probably still living. If you search on Ancestry with Hughes as the surname and Meier as the maiden name you will find them. The boy appears to have married in 1984 in Surrey. It is very likely that you could find and address for one of the children and write to them explaining what you think is the connection and can they help with your search. They may or may not reply.
I cannot see a Leslie Hughes with a mother's maiden name Heinz anywhere neither can I find a marriage of the couple. I have tried the Armed Forces marriage and British Overseas as well.
I would be inclined to purchase William and Eva's marriage certificate which will confirm that he is Leslie's brother and hopefully with the information on the certificate will lead to his birth registration and hopefully to Leslie's birth registration.