I have started with a company that employes a lot of sub-contractors. Each month they are asking for a "rolling" monthly AIR figure in comparison to the HSE average AIR figure for construction (that being 330, as taken from the Home builders federation statistics ). my question is,, is this possible to do by pro rata'ing the 100,000 by 12 ?

Therefore the 100,000 divide by 12 ,  this would be = 8333. -- the company has had 3 x minor accidents during January and we have had 500 persons work for us during January . 

Working this our would be 3 / 500 = 0.006 , times this by the pro rata'd rate , being 8333  = an AIR of 49.9.

If we then divide the HSE AIR figure of 330 by 12 it gives 27.5. therefore is it fair to state that for January our company had an AIR statistic of 49.9 against a monthly AIR from the HSE that would be 27.5 

I hope that makes sense,,  apologies if not as getting very confused 

I'm with Brian on this.  To me a rolling figure means the up to date figure for the last 12 months, regardless of calendar year.

So each accident drops out of it after a year has elapsed.

The "monthly" bit doesn't mean the accidents in the last month and doesn't involve any pro-rata, it just means you work it out each month, for the preceding 12 months.

So once January is over, you work out the figure based on accidents in the period last February to January just gone.

Once February is over, you do the same thing for last March to February just gone.

And so on.

You need a nice spreadsheet that will automate this.