Question:
Where Can I find information online about: retrofit fire alarm system selecting the suitable fire alarm sensors and control panels write the spesification for the fire control panel

Answer: not sure what you mean by "retrofit" what type of building are you talking about ? if the building currently doesn't have a fire alarm system what has changed to demand one be fitted ? what has local fire officer got to say about the level of protection required ? have you written a Fire Risk Assessment ?

there are a number of control panels on the market, manual or automatic systems (dependent on the level of protection reqd), detector type(s) to be fitted are dependent on the area they are to be fitted, (do you know how many/how far apart they should be ?), sounders come in a number of styles with as many output levels. wiring should be in fire resistant cable such as MICC, FP200 or similar do you want to have a connection to a call centre during closed hours ?

Oh to hell with it - just call in 3 local fire alarm companies and ask them to do a survey/quotation and select one of them for the job, it may be the cheapest or the highest - but make it the one you feel happiest about and who gives you advice on cost of maintenance contract (another requirement)

You might also have to worry about safety signs and emergency lighting as well as making allowance for the disabled - so all of this will be covered by about 3 Acts of Parliament and 7 Regulations - stiff reading before you start learning about the design of a Fire Detection and Alarm System