Hi there Ollie,
Are you sure that William Robertshaw had a headstone? Not all families did.
I live in Cullingworth and am a member of the village history group. We have the transcripts of the Memorial Inscriptions that were recorded by a Mr Trolley, a gentleman who died a few years ago.
He did not record a William Robertshaw, though I have seen the entry in the parish register.
The only Robertshaw Inscriptions are:
"In loving memory of the children of Alexander & Maria Robertshaw of Stony Royd, Halifax.
Jesu Merci In loving memory of Alexander Robertshaw, born January 1834, died August 1894. 'We cannot thy purpose see, But all is well that's done by Thee'.
In loving memory of Maria Robertshaw, born January 1832, died May 1887. 'I know that my Redeemer liveth'
The gravestones at the front of the churchyard are reasonably well kept and legible. However, many of those at the rear of the church are rather more inacessible.

Deetot