NHBC advise flashings “should be tucked 25mm into a brick joint and wedged in place at not more than 450mm centres, or a minimum of one per step for stepped flashings”
(I realise that is not a stepped flashing but the principle is the same and arguably more important to have it detailed into the masonry)
nhbc-standards.co.uk
Just checked the lead association, they say the same (most likely where the NHBC guidance came from)
(I realise that is not a stepped flashing but the principle is the same and arguably more important to have it detailed into the masonry)

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Just checked the lead association, they say the same (most likely where the NHBC guidance came from)