In The Flag and Snag double bill, Foggy even has to spell his name when he is collecting his parcel.
Man; How do you spell Dewhurst?
Foggy; D-E-W-H-U-R-S-T. How do you spell it?
Not only is this a brilliantly piece of sarcasm, and confirmation how Foggy spells his name, it also implies there should be no other way of spelling this name.
It seems 'Dewhirst' was only used in Foggy's second stint in the show.
I don't like the spelling of Dewhirst, it seems to posh for Foggy.
It would be interesting to see Roy Clarke's original scripts for the spelling used in all of the mentioned episodes.
I can only conclude someone involved in making the 90s episodes had their own ideas how Dewhust should be spelt.