ha, looking at your introductory post and location i think many of can guess the system you are looking to enter. as i said, measure it! but as I'm feeling nice I'll help further: I think the smaller set of keys (as described by mike) fit the hole/ slot at the peek of the triangle, of if not you can at least use the thin end of the handle to some use on it, it has been known for people to use other DIY methods on those lids (including pliers!?! i used a DIY key). on that system some of them have the two sides of the triangle bolted together so the lid must be lifted out, it will only hinge when the sides are independent (dont know if you nead the larger keys for this, there not the heaviest of covers by any means). but just go and try!!!
as always make sure you dont leave the hole open and unattended, either barrier it or close it behind you -if it is hinged, but check you can open it from below first (with someone still on the surface), covers are a lot heavier from below.
make sure you follow other safety stuff, telling someone up top where you are going, how long you will be (longer than you expect, I'm sure time goes faster underground) etc etc. -theres plenty of stuff online you can start by reading.