I am managing a HMO which has regular alarm certs. I have attached photos of the heat and smoke alarms which show no manufacturer's date or expiry.

 

I belive these to be well beyond 10 years and should be replaced.

appreciate any help 

There is no regulation that says that fire detection devices must be replaced after 10 years, and when tested so long as a device works as it should it still complies, however it is also recommended that the manufactures instructions should also be noted. In the case of your System sensor heat detector and your Honeywell smoke detector both state "They can not last forever, and we recommend replacement after 10 years" I have seen detectors that are over 10 years still work when tested.

There is of course nothing to stop you paying to have them all changed any time you wish.

Independent tests by the Building Research Establishment indicate a typical 25 year life (varying by type of device) for detectors and it would be unwise to exceed this although, as long as the devices function during service tests & aren't causing false alarms, there is nothing to make this mandatory. 10 years, often stated by maintenance firms, is purely the warranty period of some manufacturers.

I installed a fully compliant BS5839-1 category L1 system in my own house in 1990. I did it because, as an electrical contractor at that time, fire alarms were part of my stock and trade. Gent system. Apart from one device fail a few years ago, all still fully functioning. 
Many of the installations we installed around that time I know for sure are still passing muster. In fact, I would respectfully suggest that the equipment we installed at that time was much more robust with better longevity prospects than some of the more modern stuff.