Virgina has asked me if we can find out if Maud and Percy had any children:
difficult to say with any certainty using FreeBMD 
http://www.freebmd.org.uk/cgi/search.plMaud & Percy married 1915 in Lambeth if they stayed in that area there are over 20 Johnson birth entries after Dec  1914 with 
mother’s maiden name of Hall various registrations districts in London 
I couldn’t tell you if any are the children's parents are Maud & Percy Johnson you would need to obtain the certificates
Sometimes children are named after family members ie grandparents, sisters, brothers 
Details have been posted for Maud's family on the census, so will see if I can find anything out about Percy John Johnson on the censuses to see if he had any siblings 
ADDEDLooking at the list of names on FreeBMD not sure that Percy's family will help  

From 1915 marriage 
Percy father named as 
Alfred occ Ham & Beef Dealer so this looks as if it’s father & son on 1891c & 1901c
Alfred Johnson 36 Stepney 
Ham & Beef ShopEliza 36 Norfolk
Robert 12 Balham Surrey
Maud 8 “
Edith 6 “
PERCY 4 “Alfred 16 Suffolk Clerk
Herbert 2 Balham Surrey
1901  – 494 Kings Road (Ham & Beef Shop)
Alfred 46 
Ham & Beef Shopkeeper , Eliza 45 Gt. Yarmouth, Catherine 23 Hampstead, Robert 21, Maud 18, Edith 15, 
PERCY 14, Herbert 12 & Hanley 8 Woodfreen London
1881
Alfred J 26 Yarmouth 
Butcher , Eliza 26 Yarmouth, Alfred H Yarmouth 7, Catherine J St Pancras 4, Robert H Balham 2, Herbert P 2 mths Balham