Such an action would come as no great surprise to regular listeners to Praise or Grumble, Radio Sheffield's football phone-in. There, Chesterfield fans have established a formidable reputation as world-class grumblers. Faults are found even in famous victories, and the run in the Cup has been accompanied by a stream of complaints about ticket allocation. Indeed, if the enduring conundrum that is a football club's relationship with its fans is ever chronicled in a book, Chesterfield deserve a chapter of their own.
Not that Thomas, a history teacher at a local school, sees himself as a moaner. But, for nearly three years now, he has passionately taken Duncan to task for playing a defensive, long-ball game that has brought some success but precious little entertainment.
Writing in the excellent Crooked Spirite - a fanzine in the traditional mould, lots of densely packed type, a couple of photocopied pictures with captions on them, and not a sheaf of glossy paper in sight - when the Cup run got under way, he rashly predicted Chesterfield would defy history and make the third round of the FA Cup. They had done so just 11 times in the previous 45 years, an achievement that would in itself make the season memorable.
Yet with the club now three rounds further, Thomas sees no reason to temper his views. "I am not going to be bought off by a Cup run," he said this week. "It's such a patronising attitude: we'll have a good Cup run, so everything's OK. Well, we have to watch them for the other 50 or so games a season, and the football is terrible."
Would he like to see Duncan sacked? "That's very difficult, because who's to say we'll get anyone better. There are so many lemons out there. It's not just Duncan. It's the board that would decide and we have no faith in them."
Tell Thomas that Chesterfield, who are 11th in the Second Division, have the third best defensive record in the League, and he counters by pointing out that they are the third lowest scorers in the Nationwide.
Nor is he appeased by the second-half performance which disposed of Nottingham Forest in the fifth round, and impressed a watching nation on Match of the Day. "We kept the ball. If we could play like that every week there wouldn't be an issue."
Stuart Basson, Thomas' predecessor in the editor's chair, shares his opinion, but both acknowledge that there are plenty who take the contrary view in a controversy - at times more Watergate that Saltergate - which frequently finds its way on to the Internet.
Mick Sargerson has yet to engage in that debate, but at 67 the retired engineer represents the case for Duncan's defence. "I don't think some give John Duncan the credit he deserves. The team hasn't been given enough credit for being intelligent and good enough to play according to the tactics that the manager decides," he said.
Sargerson, who has followed the team since the 1930s, was at Bolton when the First Division leaders were beaten 3-2 in the fourth round. "I have never been so proud of the Chesterfield team as I was that night at Bolton."
Cup fever is a strange malady, though. On a bright spring day this week there were few overt signs that the local football club were approaching the biggest game in their history.
The ribbons in the butcher's window, so beloved by television producers, were conspicuous by their absence, as were youngsters wearing Town shirts in the busy Market Square (they are still known as "Town", long after the name was dropped from their official title). Yet, in conversation, it does seem to be the talk of north Derbyshire. There is a special edition of the local paper, the team have produced a cup record and the demand for tickets has prompted the club to bring in extra staff.
The decision to move the game back to Sunday morning, at the request of the police, has caused a certain amount of consternation, as has the fact that it will only be televised live in Wales (some of the ticketless are planning a day trip to the Principality).
Come the big day, though, most of the Sunday leagues will kick off at nine, though - disappointingly - the Solemn Mass at the Parish Church of St Mary and All Saints, whose crooked spire is Chesterfield's most notable landmark, goes ahead as normal at 11am.
To the question as to whether Chesterfield is a footballing town there was a mixed response. Thomas felt it was, and that in recent times he detected more interest from the children at school. Sargerson was not so sure. Gates are around 4,000, up on last season, but well down, for instance, on the 12,000 or so that saw the Fourth Division championship won back in 1970. No one doubted that the closure of all the local coal mines - once there were 27 pits in the surrounding area, now there are none - had an impact.
In the club's bar Roy Pollard, himself an ex-miner, said many could no longer afford to go. "Once upon a time it was five to 10 bob to get into a football match. Now, if a father goes with his son, it's the best part of 20 quid. The '84 strike decimated this town. Everybody was penniless. It broke up marriages and broke up communities and affected the football club."
As the team's unofficial historian, Stuart Basson is able to lend Chesterfield's current success a little perspective. There was a brief golden period from the mid-30s to 1951, when as members of the old Second Division the likes of Newcastle, Manchester United and Spurs were regular visitors. But for the last 40 years the club have bounced between the Third and Fourth Divisions (or Second and Third in the post-Premier world) and, FA Cup aside, they must have set some sort of record in the League Cup in all its guises, with a best-ever run to the fourth round in 1965.
The best Chesterfield player of all time, Basson reckons, was not Gordon Banks who played 23 games in 1958, but Herbert Munday, a regular goalscorer around the turn of the century. "Unquestionably, if he had played in the First Division he would have played for England."
The club's most flamboyant character, in Basson's view, dates back even further. By the time the goalkeeper Charlie Bunyan joined Chesterfield in 1892, he had already famously conceded those 26 goals when playing for Hyde United against Preston.
His fame did not end there. "He was always in trouble with the authorities when with us. He ran a pub where the fans used to congregate, and used to tell them what was really going on."
The consensus this week was that, of the modern Chesterfield teams, the present side are better defensively but not as good overall as the Arthur Cox side which won the Anglo-Scottish Cup in 1981 (beating Rangers on the way), though better than the one whose 4-4 draw at Anfield hastened Graeme Souness's departure.
Wrexham, who have a good record against Chesterfield in recent years, are feared far more than Forest, who had been beaten in a pre-season testimonial and were considered ripe for the taking. The absence of Darren Carr and Kevin Davies, sent off in the brawl with Plymouth 13 days' ago, is bound to affect them - especially Davies, who is the club's most skilful player. He scored a hat-trick at Bolton.
To have any chance, it looks as though John Duncan will have to inspire another rearguard action. If he does, however, Craig Thomas will not be complaining. "The Cup is different," he said. "I don't care how we win in the Cup. We'll leave the cerebral stuff until later."Reuse content