In our town we have a Haulage contractor with a HGV Test station, within that station they have a rolling road, this is used to check the speed of the HGV to calibrate the Taco, If it's possible for you to go to one of these places they may at a small charge check your speed for you.
For you people out there, there is a item on the market that warns you of all speed areas, its called Indica8tor with active intelligent voice guidance, & it's UK Legal, & before the do-goders pipe up, we can all do that little bit over the speed limit especialy when the Speed camers are close to each other as they are in Blackburn Lanc's. these just rest on the dash.