David (24) and Hannah (23) were found murdered on a beach in the diving resort island of Koh Tao in September.
Two men were arrested and charged with murdering the pair and with raping Miss Witheridge – but the police investigation has been a controversial one, with allegations that the suspects were beaten and tortured, that evidence was mishandled, and that police were keen to avoid charging any Thai national with the crimes.
At one point, UK police travelled to Thailand to assist in the investigation.
The defendants – Zaw Lin and Wai Phyo – could face the death penalty if they are convicted.
In evidence yesterday, the head of the Thai forensics institute, Porntip Rojanasunand, said that her team had analysed the garden hoe that was believed to be the murder weapon. While DNA from two men was found on it, it did not match the defendants, she said.
David was in Thailand on holiday after completing his degree in Civil and Structural Engineering at Leeds University – he was en route to a six-week work placement with a mining company in Australia, and was due to return to his studies for a Masters course.
The trial, which is taking place in the Thai island of Koh Samui, continues.