www.firesafe.org.uk/ a good starting point before delving into detail Quote
Audible Alarms
One sounder should be located near the control panel or entrance on a separate circuit. Addressable systems should be wired from the control to a sounder protected by a short circuit isolator.
All the sounders should sound similar to avoid confusion.
A minimum of 65 dB is required in general areas or 5 dB above any background noise which persists for more than 30 seconds.
Were high noise levels exist, visual indication such as strobes may be required.
Where sleeping people are to be woken then 75 dB is required at the bedhead.
A loss of 30 dB per door should be allowed for, to guarantee 75 dB at the bedhead a sounder per bedroom is recommended.
For areas where there are people with impaired hearing the approval of devices for people with impaired hearing would be the subject of consultation with the users.
The decibel level decreases -6dB from a sounder every time the distance is doubled as per below. This is just a general rule, this does not take in to consideration doors absorption (warehouse full of cardboard boxes) and weather conditions,

I agree with Kate, are you sure your office is 86dB's? Our workshop does not register a TWA of that let alone our offices! There may be one piece of equipment which is capable of making peaks of this noise but if this is an average I would question it. I would recommend getting a noise survey carried out by a competent company who should also make recommendations for it's reduction if necessary.

I strongly suspect a typo for 68 dB - let's hope so.

Sorry folks but I seem to have caused some confusion. The dbs are not above 86. What I meant was if Noise reg have a low level of 86dbs why should a fire bell have a lower noise db of 65? This would mean that if you had db levels between 65 and 86 you may not hear the fire bell. DB levels thankfully are not normally at this level. I have contacted the Fire & Rescue Service who have advised if there was no issue before removal of the faulty alarm bell then it should be reinstated. They have advised that what is required should be determined by the findings of a Fire Risk Assessment. They were shocked to learn that the maintenance company had removed the alarm bell without any consultation.

Many thanks for the replies and apologies again for any confusion created.