IT Magement Service Tool

We have around 300 staff. We have about 15 IT staff. Our service desk is located at our London office. We process around 300 tickets (incidents and service requests) per month. We follow ITIL processes, so the tool needs to adhere to ITIL principles and support those processes. In addition, we currently use our current tool for Change and Problem management, including asset management of laptops and mobile phones. We are looking to procure a Software as a Service (SaaS) ITSM Tool to meet our requirements.