First Light: Dramatising the real Battle of Britain
The book is his memoir of what it was like to be an 18-year-old Spitfire pilot thrust into the gut-wrenching, ear-deafening, life and death struggle of the most violent aerial combat ever.
And it deals with his mental disintegration in 18 relentless months on the frontline.
It was always going to be challenging.
This was one film where we had to get not just the emotional thrust right, but also the historical detail. There are a lot of people out there for whom this really matters - and I am one of them.
The conversations started early about getting Spitfires airborne. But what is it they say? Never work with animals, children... or vintage aircraft!
We were discussing a scene in which 'Boy Wellum', the hero of our story, makes his first flight in a Spitfire and our actor, Sam Heughan, couldn't wait to get into the air.
The problem was how to convince the audience he was actually at the controls of a Spitfire rocketing through the clouds. The big snag was that there was no way we could get Sam airborne in a real Spitfire.
This scene was crucial to the story, appearing little more than 10 minutes after the opening of the film. We had to produce a sequence breath-taking enough to make the audience believe that flying the Spitfire was love at first sight for Boy.
We had access to a real Spitfire - and the budget for maybe 45 minutes flying time - but the Spit is a single-seater and there was no question of anybody but a very experienced pilot taking the controls of several million pounds' worth of vintage aeroplane.
We had access to a replica Spitfire, which could be shoved about on the ground but had no proper cockpit interior.
This was a huge task in itself, going through around 50 hours worth of unused and unseen material, but it was great that we could give some of this footage the light of day at last!
It is lovely stuff but the registration numbers on the side of Spit in the movie footage didn't begin to match our real or replica planes.
One plane was brown and green, the other brown and grey. And the real one was based at Wycombe air park and our replica was 80 miles away on the drama set outside Dunstable.
Bringing the replica down would nuke what little was left of the budget, but if we didn't, Sam could be walking in the rain to the replica on one location and then climbing into the cockpit in bright sunshine on the other.
It was quite a headache!
Somehow we wangled it in the end. The owner of the replica was persuaded to bring his baby to stand side-by-side with the real McCoy.
Then we found a friendly pilot, prepared to have the back cockpit of his two-seater Russian YAK trainer converted to look like a Spitfire cockpit interior.
Sam leapt in, surrounded by high defintion (HD) mini-cams and took to the sky with his script taped to the instrument panel.
Meantime, our real Spit took off with the pilot delivering Boy Wellum's point of view (by way of a specially designed camera mounting on his flying helmet).
When we got into the edit, the whole story came together. Combining Sam walking to the replica Spitfire, the real thing taxiing, then Sam in close-up in the back seat of the YAK. Then cutting to his point of view shot in the real Spit, we get the hair-raising images of take-off.
And once he's airborne, we start to inter-cut Sam in the cockpit with the footage from the Battle of Britain movie.
That was the easiest of the flying sequences in the film!
Then we had to work out how to create a full-blooded dogfight, and a nightmare flight in torrential rain over the channel - during which Boy shoots down a German bomber. These scenes were whole other cans of worms...
Looking back on it all now, I can't believe we shot the whole drama, including the flying, in just nine days. We couldn't have done it without the orchestration of the first assistant director Chris Carreras, whose experience spans the Bourne movies and United 93.
He was dead right when he took one last long look at the schedule just before we began the shoot and, having considered the weather and all the other infinitely frightening variables, commented dryly: "We're going to have to be 100% lucky on this one!".
Geoffrey Wellum didn't have time to visit us on set - but before the shoot, as I was scripting, we spent a huge amount of time together. And afterwards, during post-production, Geoff worked very closely with the CGI artists to make sure we got the tracer fire absolutely correct in the air battles.
Working so closely with Geoffrey has made First Light a unique experience both for me as a director and I think, for the audience.
The combination of Geoff's expert eye-witness guidance and actually getting Sam up in the air - instead of in some faked up studio cockpit - has made the film an incredibly rich experience for everybody.
And, I guess, is just about as close as any of us would want to get to the nerve-jangling terrors of air combat, Battle of Britain style.
For me, creating the tension on the ground was just as important as in the air. I love the waiting scene in dispersal before Geoff's first combat - the tinkling of teaspoons in cups, the rustle of a magazine, Kingcome chewing on his match... and then the sudden shrill ringing of the phone - scramble!
Geoff watched these scenes with great interest and said that he felt the film perfectly caught the mood and emotions he felt at the time, both on the ground and in the air.
The war literally tore Geoff's emotions apart. If he had not been rested from flying before going back for a second tour of combat, I think he would be the first to say he would no longer be with us now.
But at that time, I'm sure, as he reflects in the film, he was desperate to fight on until the bitter end.
This was the truth for many soldiers - the feeling that they had been taken off the line before the 'job was done' and now were to be left to watch others die whom they could no longer help or protect.
Geoff still carries a sense of guilt that he survived when so many he knew died.
Geoff hates to be called a hero but his effort and that of those all around him 70 years ago, saved us from the terrors of Nazi occupation. I believe that his war - the Battle of Britain - was the key turning point of World War Two.
If England had fallen to Germany, the country could not have been used as the launching point for the D-Day landings and the liberation of Europe.
I salute you, Geoff - however reluctant you are to be called a hero. I salute you and all those that fought alongside you. And I'm sure the audience will, too.
First Light is on BBC Two at 9pm and BBC HD at 10.30pm on Tuesday, 14 September.
First Light is part of the BBC Battle of Britain season.