Prof Ged Martin looks at a Rainham door that dates from the time of the Peasants’ Revolt

When Rainham's Norman parish church, St Helen and St Giles, was built around 1170, a small doorway was inserted in the south wall of the chancel. This gave the priest direct access to the altar.

You'd hardly give the narrow timber door a second glance. Its four vertical oak planks are about seven feet tall, roughly hewn and weather beaten.

But in 2009, scientists found that they had an interesting story to tell.

Dendrochronology is the science of dating timber. The term is derived from the Greek words for tree, time and measurement.

As a tree grows, each new layer of bark forms an annual tree ring. In warm wet seasons, trees grow faster than in cold, dry summers.

By comparing the rings from felled trees, it's possible to compile growth patterns over hundreds of years. These are then used to date individual samples.

Miniature cores were drilled through the planks. The gaps were blocked with stained oak pellets.

Dendrochronology is impressive, but it can't be an exact science.

It's easier to date whole logs than sawn timber, because nobody can know how much has been shaved off the planks to square them.

Legend claimed that the famous timber church at Greensted near Ongar had been erected by the Saxons in 1013 AD.

But tests showed that the logs were felled fifty years later, around 1063. Allow a few years for the timber to season, and it now seems that Greensted church was erected by the Normans, soon after 1066, using local carpenters who built in the only style they knew.

The Rainham planks came from trees felled after 1379. But how long after? Skilled carpenters would have trimmed away the soft outside layers, called sapwood, from the tree trunk, leaving the hard innards to be cut into planking.

Scientists reckoned that they should add somewhere between eight and 24 years to guess a felling date. This gave a range from 1387 to 1403, when Richard II and Henry IV were on the throne.

There aren't many local clues from so long ago. The timber roof inside the chancel of Rainham's church has fifteenth century workmanship. Its sturdy vertical supports called king posts seem to float down from the ceiling.

Were the two jobs done together? Unfortunately, we don't have a precise date for the roof. More important, it was obviously built by experts. The door looks like rough planking just knocked together.

There's another intriguing possibility. In 1381, Essex people rebelled against an unfair Poll Tax. Sent to Brentwood to restore order, leading official Sir John Gildesborough narrowly escaped with his life.

Gildesborough owned a mansion at nearby Wennington. His property was targeted during the Peasants' Revolt. A rebel rabble probably marched down from Brentwood. Perhaps they wrecked Rainham church on the way?

All we know is that the chancel doorway was built around 1170, but the door itself had to be replaced 200 years later.

But there was one more surprise. Obtaining oak planks would be easy, wouldn't it? A few miles to the north of Rainham, the higher ground was thickly wooded. In some places, it still is.

Records show that medieval kings often allowed churchmen and other loyal supporters to cut oak trees from the royal park at Havering-atte-Bower palace.

However, scientists found that the timber in the Rainham door came from the Baltic. Nobody expected that.

Small ships sailed from Rainham as far as Calais in the later 15th century. There was a boat builder working on Rainham Creek in 1533. Evidently, Rainham's overseas trade was older (and wider) than anybody had guessed.

We think of globalisation as a modern phenomenon. It's a shock to learn that, 600 years ago, it was easier - and maybe cheaper - to source quality oak timber by ship from Sweden or Finland than to drag it from Warley woods or Epping Forest a few miles away.