1. An apparatus for monitoring a structure against intrusion comprising: a sensing device for producing a detected signal for determining a change in or to the structure; a processor for processing the detected signal to distinguish between noise in the signal indicative of a nuisance event and an intrusion event; wherein the processor is configured to analyse the detected signal over successive noise detection time intervals, each having a plurality of successive time block durations, to derive, for each block duration, a count of the number of level crossings, the count being the number of times the signal crosses a noise level threshold in a direction during the block duration, regard the signal as indicative of a nuisance event upon the count in each time block duration of a noise detection time interval being within a noise amplitude variation range, the noise amplitude variation range being a permitted range of variation in the number of level crossings per time block duration, generate an alarm upon the count being in excess of a current event threshold in any time block duration, the current event threshold level being a number of level crossings per time block duration that indicates that an intrusion event has occurred, the current event threshold having been set as a number of level crossings in excess of a maximum number of level crossings within the noise amplitude variation range that occurred within a noise detection interval, maintain a current event threshold upon the count in a time block duration of a noise detection time interval being outside a noise amplitude variation range and below the event threshold, and set a new event threshold to apply in a subsequent block duration upon each count in any time block duration of a noise detection time interval being within the noise amplitude variation range.