Fans of Edith Pargeter - who wrote the bestselling Cadfael Chronicles under the name of Ellis Peters - will be making a pilgrimage to Shrewsbury Abbey to see a new stained-glass window commemorating the creator of the medieval sleuth. The memorial window will honour St Benedict - Cadfael was a Benedictine monk - and will get the Bishop of Shrewsbury’s blessing on September 14.

By then, fans will have seen three new episodes of Cadfael on ITV starring the tonsured tec. Like the previous series, they have been filmed in Hungary, where Cadfael’s 12th-century Shrewsbury has been built in studios just outside Budapest. Low costs and abundant forests make it a good place to recreate medieval England.

What’s wrong with filming in Shrewsbury? “The problem,” says producer Stephen Smallwood, “is that it’s a traffic island”, with its medieval elements choked by roads and railways. The rot set in when “Thomas Telford put what’s now the A5 right through the cloisters of Cadfael’s abbey.”

Smallwood rejected Shrewsbury Abbey because “although it has a Norman nave, its interior is cluttered up with relics of subsequent centuries - what we did was model the inside of Cadfael’s abbey on the Tower of London chapel.” Respect for historical accuracy runs in Smallwood’s family: “My father was a Church of England canon and an expert in church architecture.”

It’s hard to keep the 20th century off the set. “Once we had the Great European Air Race taking place while we were filming - planes zooming 60 feet above the abbey are not very medieval.” Another time, the Hungarian Grand Prix took place just down the road: “Just as Derek Jacobi - who plays Cadfael - is getting up to speed, you suddenly hear Michael Schumacher roaring around a bend.”

Historical accuracy is even harder when it comes to people. Smallwood’s research shows that many medieval people wore the same colour and texture of clothing and that most men had beards. “But if you film all the characters looking almost identical this is wildly confusing to a television audience - so ‘ve trimmed hair and banned beards.”

As regards costume, Smallwood has “colour-coded the characters to make the plot easier to follow”. There are other limits to authenticity: “By the rule of St Benedict, monks were required to lower their gaze in the presence of a woman - which wouldn’t make good drama - so we’ve abandoned all that.”

On top of this, fans of the books complain about the adaptation. “People are livid whenever we change the plots,” says Smallwood. Or the dialogue: “Edith Pargeter, who died last year, was disappointed that we didn’t use her dialogue, but it’s too elaborate for television. And, anyway, not historically accurate: in the 12th century, the nobs would have spoken French and the peasants a Saxon argot.”

Smallwood praises British audiences as “very historically aware - they enjoy pointing out mistakes”. One person wrote in saying he’d heard a collared dove on the soundtrack, which was wrong because the species wasn’t introduced to Britain until the 19th century.

While Cadfael’s designers wrack their brains about whether medieval carts had solid wheels or spokes, and Smallwood searches in vain for mules in Hungary, Derek Jacobi adds to their problems by insisting on a genuine tonsure. This means all the actors who play monks have to have the same haircut and they “creep around Budapest terrified of being mistaken for clerics”.