Stuart, I beleive the model number you have is Rinnai, Model 151 Portable Gas Heater (Meaning it's flue-less). If that is the case, the Oxygen Depletion Sensor (ODS), which is part of the Pilot, may be clogged with dust, lint, pet hair, etc. In which case, cleaning it, with canned compressed air with a straw nozzle (like you clean your computer with) can solve the problem. Also, cleaning the entire interior of the heater is a prudent thing to do, at least once a year. A vacuum cleaner that has a hose and brush attachment works well for this. To assist you, I have included a link to "How to clean your Pilot and Burner Assembly" Demonstration, as well as a link to the Owner's Manual for you Model Rinnai Portable Gas Heater.
Cleaning Demonstration: http://www.desatech.com/techDocs/Cleaning%20Demonstrations.pdf
Owner's Manual: http://www.rinnai.com.au/images/stories/operating_instructions/Heating/Portables/Portable_Radiant_Convectors.pdf
I hope this has helped you troubleshoot and solve the problem. Please let me know. Cheers!