On Mary's marriages certificate, she states her father's was John - occupation Mason.

Just to confuse things....

1841 census, Broad Holes, Lydney

John WALKER - 40 - Mason (born in County)
Sarah - 43 (think this is John's second wife) *
John - 15 - baptised 5.8.1827 - John and Ann WALKER - occupation Mason
Mary (?) - a faint second initial - age 13

Marianne WALKER baptised 5.9.1829 - Lydney - John - occupation Stonemason - mother Anne

possible marriage:

John WALKER (Widower) married 'Sally' Maddox - 8 June 1834 Lydney

You will also find on the FOD records

John WALKER married Anne TEAGUE 18 October 1826 in Lydney: Witnesses: * William SPIERS and Mary SPIERS

Which I think connects with census information on this subject

So we have two brothers: John WALKER : christened Westbury on Severn 28 February 1802 and James WALKER - Westbury on Severn 1799 - parents John & Sarah WALKER