The fire, which broke out at around 2pm, was contained to the roof area of the factory, at the Chiltern Park Industrial Estate, and all persons were accounted for.
The TRU, equipped with ropes and other rescue equipment, was sent to support crews working at height.
Some crews have already left the scene leaving others to assess the building damage and also ensure that the scene is left safe.
>This is the second roof fire at the factory after fire broke out at the premises two years ago.
Emergency services rushed to the burning unit on April 22, 2020, fighting flames which spread up to the warehouse roof.
That fire started on a plastic oven conveyor belt.