Okay. I wish I could be of more help, but it's difficult to pinpoint with 100% accuracy whether or not any combo is safe.
You do have 2 things working in your favor:
The diazepam will help to even out any seizure risks from the tramadol, although the tramadol is such a small dosage I think the seizure risks are pretty small.
And codeine is a relatively weak opiate, especially if you stick to the 30mg as prescribed.
If you've been on the diazepam for a while and have a tolerance, I would hypothesize that you would be okay. But this is speculation. There is just no way to say for sure.
I myself would take it, but I have such a high tolerance to benzos and opiates that I would take my chances, knowing the potential risks involved.
Honestly I would call your doctor or pharmacist to get an all clear. I'm just not comfortable telling you it's safe.
Sorry for the long "non-answer" :/