No central heating or hot water?

Oil fired Boulter boiler. Starts up from cold and runs for approximately 5- 10 minutes then stops. Fitted new circulating pumps last night, water circulating but only slightly warm before boiler stops. Motorized valves opening fine. Boiler smelly when the front screen is removed. Could it be the thermocouple? If so is this difficult to replace?
Thanks in advance Bill