You dare not leave the hot bulb lurking in the soot. It could end up in the combustion chamber and do untold damage. Your only sensible option is to draw the head and remove the bulb properly. BMC 1.5 heater plugs should be changed or cleaned annually to minimise the risk of this problem. When withdrawing the plugs do not just unscrew them; work them in and out to free and disperse carbon deposits around them.