Each of the scripts were based on real events and thoroughly researched. On several occasions throughout the series's run, the BBC would reject a script as it was deemed too accurate and potentially upsetting to audiences or too politically sensitive.
Belgium - noted for its flat landscape - was recreated on locations in East Anglia which is similarly flat, though some exterior locations were also filmed in Brussels itself.
Gerald Glaister pitched the series to the Controller of BBC1 when he shared a lift with him at the BBC Headquarters. By the time the lift reached the Controller's floor, Glaister had himself a deal.
The BBC was dissatisfied with the final (43rd) episode, possibly due to its polemical tone, and claimed it had been unable to complete the editing due to ongoing disputes with the broadcasting unions. According to the BBC documentary Shelved, broadcast by Radio 4 on 12-12-09, this was at best disingenuous and in fact the script's leaden anti-communist theme was felt to be at odds with the series' previously subtle characterization. None of the cast members interviewed for that programme were now in favor of it being released.
Series creator Gerald Glaister served as an RAF pilot during the Second World War and survived over 100 sorties over enemy lines.