Crikey. [no ofence!] I can see why you're worried..i'm not an expert yet on breeding so it would probably worry me too. My first clutch hatched at exactly 52 days, which I had counted as as their 'due date' by reading Kathy Love's book on corns. So they were bang on. What are your temps in the incubator? Sound like it might be a bit cold. It should be between 28-31C ideally, with higher temps being more dangerous than drops in temp. If its too cool then your eggs have not been killed but they are just not developing at the right rate. I used a simple homemade poly-box incubator with het mat stuck inside the lid, with eggs in a tub of vermiculite. I got it on a thermostat that keeps it a just the right temp. And they hatched exactly when it said they would, so I'd hazard a guess it is your temps.
I used a digi therometer which measures the temperature really accurately, with the probe inside the egg tub right next to the eggs.
If it is definately positively not the temps I would be inclined to slit one open and see what's going on. If temps are normal I can't see why it would be taking this long. ??? I'd want to see if the babies are developed, but I an understand if you were too afraid to do that. Id be worried if they were stuck then you can't leave them forever or the whole lot could die...so would risk the possibility of one dying. However if it is fully developed whn you slit it, just leave alone and it will poke out its nose and start breathing air...it'll be ok. If others think what I have said is a load of crap (coz I am only guessing) then please contribute!!