If you are specifying an alarm system as opposed to installing one to some responsible persons design, it rather  sounds as if you are at least in part, responsible for the fire risk assessment. That is fine, but be aware of the responsibity this brings.

https://www.london-fire.gov.uk/safety/the-workplace/fire-risk-assessments-your-responsibilities/oe

https://assets.publishing.service.gov.uk/media/64257bbe3d885d000cdadf2d/A_guide_to_making_your_small_non-domestic_premises_safe_from_fire.pdf

For example to be aware that if the escape distance from anywhere on  either floor to fresh air exceeds 25m you cannot have a simple open plan arrangement or you may need more fire exits to get that distance down,

Now to the exam question.

Don't install a smoke detector in an area where cooking will take place, or where the fumes from cooking are expected  (what sort of cafe ? Tea and biscuits, or frying and grilling ?) Heat detection, either absolute or rate of rise, depending quite where, is the way to go where there is cooking,.

Areas with significant cooking get complex quite fast - does gas need to be shut off and ventilation stopped if the alarm goes off ? Do you need to detect if there is a fire in extractor ducting - chips can make extractor pipework dangerously greasy,,, 

The basic D1 alarm may not be the right choice if it is a yes to any of those ....

Not intending to put you off, but to stimulate a bit of thought before leaping in too deep.
Mike

I am specifying it, with guidance from the local fire brigade. The escape distance is less than 18m. It is not a greasy spoon, no frying, more hipster joint, tray bakes and speciality coffee/teas etc, hence I expect a bit of humidity. There will be adequate extraction installed. No automatic gas shut of required. 

Key aspect of early detection/warning is also via the people using the building. That means appropriately located manual call points and a panel system in accordance with BS5839-1. I am surprised anyway at FRS accepting a Grade D1 system unless as some kind of temporary measure.

Any design should be predicated on the FRA/strategy for the premises, even small ones such as this. 

Yes, I am a little surprised too. But it's such a small building anyone shouting fire would be heard by occupants. It previously had no fire detection at all when it operated as a cafe by previous owners. Nevertheless, the fire brigade are satisfied that a D1 system will suffice. It is just a question of heat detector and siting of the smoke detector that is in the same space as the kitchen.

Northern Spark said:

I am to install some fire detection in a cafe. It will be a domestic type system to Grade D1.

Either it is domestic or commercial. It cannot be both.

If the Fire Service are the paid Consultant for the design, then I cannot see any problem, however, if it is one person who has visited and said, verbally, yes, that'll be OK, then I would certainly be getting another, paid for, opinion, as you are now the designer of the system, and you really do not want to be the designer unless you have adequate insurance cover, and the relevant qualifications to design it.

I'm not having a go at you, but, if you asking whether a heat or smoke detector should be fitted in a kitchen, then you are not showing any competency in the design part of the work. After Grenfell, fire safety is being taken far more seriously, please check that your insurance covers you for the design and fitment of the system, and that it complies with local authority and national requirements.

alanblaby said:

please check that your insurance covers you for the design and fitment of the system

A good point. If it does, it probably costs more. Significantly more!

Perhaps a rewording of the question. I wasn't asking if I should install a heat detector, rather, is it likely to detect a fire before a smoke detector in the same space. 

So, how far away from the kitchen should a smoke detector be installed to prevent or reasonably reduce the chances of false alarms?

No, a heat alarm will not detect a fire before a smoke alarm.

But much more importantly, it won't false alarm on steam from a kettle, of a whiff of smoke from the sandwich toaster, and if there is a real fire that is much more serious than that, it will still detect it before it gets too big.

So as it sounds like the place is more or less open plan, you may end up needing  to do quite a lot of it with heat sensors. But you need to see how the extraction works and consider where any fumes will accumulate.

Mike