Bull Sand Fort and Haile Sand fort Humber estuary | Education Video by World of Military

Now privately owned by Brian and Norma Rushworth, 

Bull Sand Fort built between 1915 and 1919, sits three miles (4.8km) from Grimsby and can only be reached by boat or helicopter.

Agents Savills said there were "a lot of bidders" for the building, which had a starting price of £50,000.

Its sister fort on Haile Sand sold at auction in 2018 for £117,000. That had a guide price of £90,000.

Steven Morish, from Savills, previously said Bull Sand required "an imaginative buyer with a project in mind".

He had described the structure as "exciting and unique" and said suggestions for its future use included "a high-end hotel, restaurant, Airbnb, Grand Designs-style personal dwelling, retreat or a tourist attraction".

The fort, which is big enough to house 200 people, was reinforced with concrete and fitted with 12in (30cm) armour, designed to withstand gunfire from battleships.

It was built, along with its smaller companion fort on Haile Sand, to protect the sea entrance to the Humber estuary, but both were completed after WW1 ended.