HMO an d has metal consumer unit in walk in cupboard. (Also used for other items. Timber does so I am recommending Supalux on inside of doors, but noticed no sensor for alarm seen here. Should it also have a sensor or at least a stand alone smoke alarm in the cupboard?

If you are going to add an extra detector, then to be of any use it should be part of the main system, not self contained. If it's a licensed HMO it's worth checking with the Local Authority that they accept an upgraded door in their fitness standards- traditionally upgrading with board as you suggest was an official method in many LA's local guides for many years but times change and it's worth double checking as standards do vary between LAs