MOST SECRET. Uncovering the Story of Northumberland’s Underground Resistance: the Auxiliary Units of WW2, by Ian Hall. Published by Wanney Books ( £4.50. Softback.

WHEN Britain was facing its greatest ever threat of invasion in the summer of 1940, plans were laid to form a secret force which would emerge from underground bases to carry out acts of sabotage and espionage behind the lines of the German forces.

More than 500 of these auxiliary units of volunteers – the British Resistance – were created, and in Northumberland alone there were more than 20. Their story has been pieced together only recently by local researchers Stephen Lewins, Bill Ricalton, Phil Rowett and the author, who have located the surviving remains of the units’ underground bunkers and interviewed a number of the auxiliaries who lived in the county.