Dimplex CXL18N?

Hi people, looking for an answer to help me out here now it's gone too cold. I have a Dimplex CXL18N storage heater that runs on econamy 7. The problem i'm having is that it keeps cutting out when it comes to storing the heat overnight. I can turn it on during the day or night and it will heat my room up but will not store it during the 12.00 - 7am period. I have located the re-set button and it works ok for that night but afterwards it cuts out again. Any ideas what part I would need to change to fix the problem. Thanks in advance. Garry