Eosinophilic oesophagitis (EoE) is an inflammatory condition of the oesophagus (food pipe) that can lead to difficulty swallowing and to food to getting stuck and has become increasingly common over the past 40 years. EoE is triggered by a protein in the diet but alterations to the types of bacteria (microbiome) in the oesophagus may also be involved. EoE is diagnosed with gastroscopy (a thin camera test via the mouth) where 6 tiny samples (biopsies) are taken. Treatment is either with removing food groups from the diet or medications including steroids (budesonide) or a proton pump inhibitor (PPI, omeprazole). The aim is to improve symptoms and to stop scar tissue forming that can cause food to get stuck. Patients with EoE will need to undergo many gastroscopies over their life, which even with sedation can be a daunting experience. There has been research into less invasive tests and two previous studies have shown that a thin swallowed string may be able to detect substances (biomarkers) that indicate how severe the EoE is. These studies were small and it is not known how the biomarkers change with different treatments or how well they relate to symptoms and findings with endoscopy. In this study the investigators will ask adults with EoE to swallow a thin string made of rayon for 30 minutes, with one end taped to the cheek, which the investigators will analyse for biomarkers and bacteria, on the same day as their routine gastroscopy and also perform a symptom survey. The investigators will also take an extra 2 biopsies to analyse the nerves which may explain why some patients have more symptoms than others. The investigators will repeat the string test on their next endoscopy, to assess what the changes have been in response to their treatments. These findings may improve understanding on how to monitor EoE in less invasive ways in future.