BRT, you aren't from these parts are you? A bivvy tent is used by mountaineers and soldiers as it is small and portable. 'Bivvy' is a corruption of 'bivouac' which came into use during the Napoleonic wars and come from the middle age German 'biwacht' meaning 'second watch'. It means a temporary camp.
under Blackpool Tower, under the circus, it's already got the mechanism and anything odd occurring up there would be totally ignored, especially between the hours of midnight and 5am as it is the norm already.