They did give the signal Cromwell on Sept 7th 1940... Oops!

September 7th 1940

The signal for this alert is "Cromwell", a code name that was used only by the Army. And one of those Army battalions that were alerted was the 18th Australian Infantry that was based at Amesbury Abbey. The report came through at about 9.30pm. The 18th Infantry Brigade was at this time, only at about half strength because half of the brigade was on leave, many of them now trapped in London because of the bombing. But was hard to keep such a secret, church bells started to peel, road blocks were set up and even plans were put in place for the blowing up of some of the bridges.
For those that were on duty, they were told to stand by for an immediate move. By midnight, no further information or orders had been received so the men were allowed to return to their billets, but were to be prepared to move at one hours notice should the invasion be confirmed.

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