TSR - the answer will lie inside Monarch's operating procedures and yes, they may well have chosen to be what you consider to be 'over-cautious' - bear in mind how long they have been operating and their 'record' - it is their choice.
Most airlines have a minimum of 1500' agl for the gear down, and 2000' in 'bad weather'. Add to that the fact that a company's SOP approach speeds/flap settings may well require gear down for handling purposes whereas another company may not use the same speeds and may thus be able to fly 'gear up' until later. All airlines will require gear down by 1000'agl.