The father, his wife - both thought to be in their 40s - and their young son, aged around 12, were treated at the scene by South Central Ambulance Service for gas inhalation and taken to hospital as a precaution. No-one was seriously injured.
Firefighters used breathing apparatus to enter the plant room. They used dry earth to soak up a small amount of spilled chemical and ventilated the room. Crews from St Mary's, Redbridge and Hightown dealt with the incident.
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Maidstone and Tunbridge Wells NHS Trust has been fined £180,000 after it burned a man who was in for a routine operation.
Thursday night's weather for the east of the region
Thursday night's weather for the west of the region