Unless you have already done so can I suggest you have a read of HSE publications HSG150 and HSG168. Both are freely available on the HSE's website. Hope this helps.

Your precautions or controls should be governed by your fire risk assessment. Under The Regulatory Reform (Fire Safety) Order 2005 you are required to carry out a risk assessment. The HSE will not just accept that you have put in some precautions. This fire risk assessment will need to be regularly reviewed as the hazards and risks change during construction. Your precautions or controls will therefore also change. Most regional and small contractors do not do this well and are running the risk of an improvement or prohibition notice at the best. If a timber frame is involve this can dramatically increase the risks depending on the size of it. If need to look at the guidance Chas has posted for further information.

In my opinion, and depending on the type/size of your construction site, you may be out of your depth. There are so many variables to everything fire related on construction sites that you really need a competent person to assist you. For example on a reasonable sized shop fitting site an alarm on each floor and at each exit would suffice, this could be a simple bell, klaxon, or battery operated break glass point. On a much larger site covering a few acres obviously a lot more is required. Timber framed buildings under construction require far more fire precautions that other types of construction, and on and on and on.

pad, there is free guidance from HSE on Fire safety in construction http://www.hse.gov.uk/pubns/books/hsg168.htm Follow the download link to the download page, look for the pdf link or use this link to download the guide for free: http://www.hse.gov.uk/pubns/priced/hsg168.pdf 95 pages of guidance. Read it carefully for what is relevant to your site.