OK - I agree, the John Walker of Cornhill is not our man.....
I think our one from Shefford died 1871 - 1876, but I've not found that...
There's another Probate record, this time dated 29 Feb 1876 for Selina Walker, widow died 17 Jan 1876, late of 82 St John Street Clerkenwell, to Richard Walker, watch maker, the son & next of kin. Estate < £100.
Her death index in Mar 1876 Holborn, London at age 64.
On 1881 at that address is Richard Walker b 1837 Watch Maker Examiner with wife Sarah & family.