Wow, thanks all for the quick advice!
Our DIY assessment was done because of the low amount of employees we have, the nature of what we sell, and the expected amount of visitors to the ground floor. I doubt there will ever be more than about 10 customers in at any one time (and hey, if there is, that will be great and we can reassess the fire alarm situation and might even be able to afford one!). However we did make a report on all the points including how long it takes to reach a fire exit and so on.
I will be brave, and call the fire service and see what they say. Oh, and see if I can get an new electrician!
I would have thought that as a minimum an M category system would be sufficient to meet most legal requirements.This is a break glass only system so that if a fire is spotted the alarm can be raised and all can be evacuated.On a personal basis I would say cover the stairways and escape routes (this would include the groundfloor areas that you have to pass through to reach the exit) with automatic detection - your insurer may ask for this or may give you a discount on,or enhance, your premium.Automatic detection throughout the premises would only be beneficial if the fire alarm is monitored (ie - it goes through to an alarm receiving centre) otherwise you're depending on someone hearing the fire alarm and ringing 999 out of hours!
With regards to Chris "not calling the fire service" the will (in most cases) only highlight breakglass units and audibility with regards to a fire system.If the public had no access to the premises then they may say you do not need a fire system if your staff numbers are minimum (not sure of exact number).However they may highlight other areas that you have missed but even then they wont just close you down or stop you trading.
Not knowing your site but have a visual in my head of what it is,the £16k does appear excessive.I do agree with Brian that most times the fire alarm is left to the end of the budget.It annoys me (as an engineer) to see multi thousand pound light fittings and marble walls being installed while the guy is bending my ear over an extra detector or two which cost a fraction of that.In your case you have to assess what you can afford to lose over what is feasible for you to spend and,ultimately,what is your objective in installing a fire alarm system.
Take my advice and contact an actual fire alarm company with regards to an installation.You can ask them for a survey ,which is free (well,it'll probably be built into your price if you agree but thats another story!!),and for alternative pricing depending on the level of cover that you require.