Retired Major General Peter Willaims worked as a BRIXMIS Tour Officer during the Cold War (Picture: Peter Williams/BRIXMIS Association).
Despite reporting on the Armed Forces for 17 years, BRIXMIS was not a unit I had ever heard of.
That was until I first met Dave Butler at the National Memorial Arboretum back in 2016 when I interviewed him and other BRIXMIS veterans at an unveiling ceremony for a plaque in their honour.
As Dave and his comrades shared with me their stories of sneaking behind the Iron Curtain and spying on Soviet troops at the height of the Cold War, I was fascinated.
A few years later I saw an opportunity to expand the idea into a documentary and set about trying to convince my editor to agree to the idea.
What helped seal the deal was when the BRIXMIS Association invited me to peruse, and ultimately use, their exceptional archive - a treasure trove of video material shot by the unit themselves and an opportunity to witness actual spies at work.
This rare glimpse into intelligence-gathering and espionage feels really unusual and a fascinating window into what is normally a hidden world.
Dave told me there was a reunion conference taking place in June 2019 in Potsdam, just east of Berlin, for BRIXMIS and USMLM, the sister American mission. We decided that was when we should time our visit.
I was also very keen for Forces News' senior cameraman Mark Hearn to film the trip.
Mark and I have worked closely on many foreign trips, and I knew I could count on him for his first-class creative eye and editorial brain.
More importantly, he had lived through the Cold War years in Germany.
Between 1988 and 1992 he was stationed at the British Joint Head Quarters in Rheindahlen and had personally filmed much of the BFBS archive material of former East Germany and the Berlin Wall.
With the crew in place, we booked flights and hotels and hit Berlin with a packed itinerary.
On our first day in Germany, we joined a coach trip of veteran spies from the UK and the United States, their former live-in staff who had once been on the STASI payroll and various other shady figures who were not keen to be on camera.
The transport took us to Zossen-Wünsdorf, also known as "the secret village", which was once home to 75,000 Russian soldiers and their families.
The Soviet headquarters and bunker complex was abandoned after Communist forces pulled out of Germany, and it was out of bounds to the public.
For the former spies, the lure of what was once hidden behind the Iron Curtain was clearly deeply exciting and rejuvenating, and for us watching them, a new insight into their work and a glimpse into how they had once operated in enemy territory.
Throughout our conversations and planning, I had stressed to Dave how much I wanted to find someone who could give us the view from the East, ideally a former Russian soldier with the inside scoop on how the Communists had viewed the foreign missions.
Straight away Dave suggested I speak to Sergei Savchenko, a former SERB (Soviet External Relations Branch) officer who had also served in Germany from1986 to 1989, the same years as Dave.
SERB were the body charged with keeping tabs on BRIXMIS and thwarting their intelligence gathering.
Making contact with Sergei ahead of the trip proved impossible and we had to fly out to Germany in hope rather than confidence.
Luck was on our side as a chance meeting in the conference corridors yielded up an agreement from Sergei not just for an interview but to spend the afternoon tramping the streets of Potsdam and walking over the Gleinicke Bridge answering Dave’s questions.
Also high on our wish-list during the planning stage was to go deep into the East German countryside to an area where Dave had once found himself cut off by Soviet troops, without a vehicle and behind enemy lines.
Desperate to make a call back to base, an unknown German landlord let him in at great personal risk.
Dave had heard that Herr Oertel had recently died but he was keen to trace the family and thank them.
The only snag was we did not have an address. What he had was a photo of the pub as it looked 12 years ago when another former BRIXMIS man had visited, and a picture of Herr Oertel himself.
Once again, we seemed to have the rub of the green and after a couple of false starts, we found the Gaststätte (guest house) that sheltered Dave from the Stasi all those years ago.
Our last day of filming in Germany took us to what was known as the Leiberose Tank Range in 2986.
The area is now part of a nature reserve, although we had official permission to drive onto the area, there was still some light fencing and three rather rabid-looking guard dogs to negotiate.
But it was worth it. Dave was so excited I could barely hold him back long enough to get the camera rolling before he was off scouring the area.
In under an hour, we found old RPG fragments, Soviet boots, and even a hidden bunker.
Back in the edit suite, the resulting footage makes for a really interesting watch and a rare chance to hear British, American and Russian voices recalling a recent but vastly different past, that after just a few decades has been largely forgotten.