Zero hours contracts can be an effective way of managing an unpredictable workload. It's a shame that the phrase now has such negative connotations resulting from misuse by major employers. It is not unusual for village halls to employ caretakers or cleaners on an "as and when required" basis, dictated by the ebb and flow of bookings.
Two minor points to bear in mind. Firstly, the employee is not obliged to accept work offered under a zero hours contract and you will have no powers to insist that they turn up at the required time. Secondly, adopting the National Minimum Wage as your rate of pay means that you will have to be very careful to ensure that you comply with every part of that legislation (travel time, waiting time, holiday entitlement et al) and do not inadvertently find yourselves paying below the NMW.