In Flag, name spelt out as Dewhurst.
Christmas Special - And a Dewhurst Up a Fir Tree.
In the 90s there's an episode called The Dewhirsts of Ogleby Hall.
Another 90s episode - can't remember the title - Dewhirst is sign-written on a van.
1983 edition of first novel states Dewhurst, as does the 1992 Country Companion book which is based on original scripts.
In Alan JW Bell's book he refers to Foggy as Foggy Dewhirst - though Fir Tree episode listed at back is spelt with a U.
It seems Dewhirst is only used when Alan JW Bell is present.
Yes, I see what you mean. However, as a convinced believer in the cock-up theory of politics as opposed to the conspiracy concept, I suspect some sub-editor or some such simply mispelt the name in the title of "The Dewhirsts of Ogleby Hall" and Bell simply followed suit in his book. If we think about it, it is not Bell's call but Roy Clarke's.
And, of course, the spelling Dewhurst is the more common.