In the summer of 1940, the question Britain asked itself was not 'will Hitler invade?' but rather 'when?'

Sealion, the German invasion plan, was actually centred on South East England, but the very real threat of invasion provoked a programme of frantic construction throughout Britain. By the end of 1940 thousands of pillboxes, anti-tank stop-lines, coastal defences, heavy-gun emplacements and anti-aircraft batteries had sprung up all over the British Isles.

It is estimated that more than 18,000 concrete pillboxes were constructed during World War Two across the UK together with hundreds of miles of defensive ditches and barbed wire. Not since the building of the 'Martello Towers' during Napoleonic times had Britain embarked on such an ambitious programme of home defence...