If you've seen the sign close to an airport, military airfield, chemical works etc, it's probably directing the Fire Service to the official Emergency Access point to that site.
Thanks all, I'm now going to be looking out for more of these - its only in the pas 2 weeks that I have seen them - with the 2nd one yesterday, the 3rd tonight.
Emergency Access
Fire Service.
The number is the number of the access point , some places have several of them. The arrow shows the direction to the access point.
I live near Duxford airfield and these signs are there , I used to know a guy who was a firefighter at Duxford .