When he was a boy Geoff Armstrong enjoyed building models. Cars, planes, tanks. Each tiny piece would be carefully brought together to form a perfect replica of the original.

Many years later Geoff is doing much the same thing.

Except now the replica is full size: an 8.5 metre-long World War One tank made of wood.

This remarkable creation will be a highlight of This Day in 1918: the event at Carlisle Castle on Saturday November 10 to mark the centenary of the Armistice which ended the conflict.

It aims to give a taste of what life was like during that momentous time, when the castle was an active garrison.

There will be recreations of a trench, a military hospital, a State Management pub... and there will be Geoff's tank.

The Mark IV was a British vehicle used during the last year of the war. One sat outside Carlisle Castle's gates until 1935.

Geoff, a carpenter from Brampton, is building his version in a workshop at RAF Spadeadam on Cumbria's border with Northumberland.

He has worked there for the past 16 years. Employed by contractor Landmarc, Geoff's main job is building targets for RAF planes to practice on.

These include replica tanks. Viewed from the air, they do not need to be as detailed as Geoff's Mark IV.

"From 10 to 15 metres away it looks quite good," he admits.

He is being unduly modest. Even much closer than that, this beast looks ready to pounce. The green-painted wood seems metallic. The tracks are painted black. It appears every inch the killing machine.

And it turns out that those scale models really are accurate. Geoff has made this monster by taking a kit made by model manufacturer Emhar and increasing the scale.

"They're very good at showing a proper World War One tank," he says.

"It gives you quite a lot of detail.

"I bought a 1/35 scale model and expanded everything by 35."

Simple in theory. In practice it requires a huge amount of skill.

"Everything's hand dressed and hand cut. We don't have machines to laser cut everything and weld it together. It's just me in a workshop with a few saws and a tape measure.

"There's nothing hard about it. It's just getting your head around how you're going to build it. Making sure it's strong enough to hold it together and light enough to lift it."

Geoff's solution was to build the tank in 14 parts. It would have been too big to fit through the castle's arch if constructed in one piece.

"Each part has to be able to be lifted by two or three men. Everything's got to sit in the back of a Transit. I have to go down and assemble it. It shouldn't take that long to put back together.

"I'm up against a time limit so I've had to design it so it could be built relatively quickly."

Geoff started this project at the end of January and has so far spent about 500 hours on it, between target-making duties.

It's made from soft wood and plywood, apart from a plastic tube for a gun, and the plaster of Paris rivets.

Mark IV tanks were classed as either 'male' or 'female', depending on what kind of guns they had.

The male had a 57mm cannon.

"The females had five little guns," says Geoff. "Just as deadly, like."

The Mark IV was big enough to accommodate an eight-man crew.

It will not be possible to go inside this version, and it doesn't move.

"I'm good, but I'm not that good," says Geoff.

A colleague has made some of the rivets and another has done some painting.

Otherwise, it's all Geoff's work, with plenty more to do before the tank's day in the spotlight.

"There's probably another 100 hours' work. One side sponson and the track to finish. And get it so it will bolt together. It's just held together with clamps now."

Will it be ready in time?

"I hope so. Everyone says 'I'd love to see it when it's finished.' So would I! Sometimes you're working out how to do something at two o'clock in the morning.

You think 'That's how I'm going to do it!'"

Geoff is not nervous about more people seeing the tank than his other creations.

"I'm happy with it. That's all that matters to me. The most challenging thing was just getting it to look right. You're interpreting a tiny little model.

"It's a representation. There will be something that won't be anatomically correct. No doubt there'll be some anoraks who will pick up on it.

"I wouldn't say I'm a military nut. I'm interested in old stuff. I've got an old motorbike I'm restoring. My father was a joiner. I used to help him out when I was young. I enjoy doing it. When I stop enjoying it I'll find something else."

For the moment, he returns to his tools, his calculator and his sheets of wood.

Geoff might finish the full-size Mark IV before he completes the model it's based on.

"That will get finished," he says.

"It's easier to measure when the bits are separate."

It seems a shame that such an amazing creation as this will be seen for one day only.

Ronnie Papaleo, a Cumbrian who builds and supplies props for films, wants to use it in a film about a World War One tank crew stuck in no man's land.

This Day in 1918's organiser Colonel Anthony Steven says: "I hope we will find a long-term home for it under cover at some point."

As for the tank's creator, Geoff says: "I'd like to see it stored somewhere. Maybe in a museum. I could take the grandkids. They're three and seven.

They would go 'Grandad built that!' I think they'll be quite impressed with it."