Brian, if it is instantly tripping, and assuming it is a new or certainly no older than 2004, then it is more than likely to be the element at fault.
Early Ultrastore boilers had a reset button, but this was removed in favour of an auto reset in case of overheat.
If the sensor is faulty, then the whole element has to be changed as well.
To check the element, trace the cable from the boiler back to its connector and after disconnecting the plug, do a continuity check on the element.
If it is the element, they cost around