In the SOAH pilot, and the first few series, David Jason used a very mild stutter as Granville, which I think was to signify the way he was becoming like Arkwright.
But in later series, I felt the stutter was not used.
It was probably be lost amongst the various plot lines that became a mish mash in it's latter years. By broadening the cast as it went on the situations got broader accordingly. Mrs Featherstone suffered badly from this and the firm but fair anchoring of Nurse Gladys was sadly missed.