I agree with the above. It's a while since I have been in a job where I got occasional random drugs tests, but from memory, I was asked before I took them whether I had taken any medication in the last 12 (or maybe 24) hours - think one occasion I had taken over the counter paracetamol + codeine that day, and this wasn't an issue.
If you don't tell them up front, and then argue that a positive result was a due to prescribed / over the counter medication, you're in a more difficult position.
Somewhere along the line, the PCV licence application will include a medical (the standards for this are set nationally) - it's a long time since I did mine so can't remember whether the question of prescribed medication came in to it.
The company's doctor may want to talk about whether there's a risk that taking diazepam may affect your driving safety (i presume you've already discussed this with your GP, as it appears to be illegal to drive if it impairs your driving - more
here.)
There's a chance the company will be concerned about taking you on with existing health condition/s, but that's a different question. Again, recommendation would be to be upfront about it all - including what you're prescribed - rather than let them find out after you're ill or any sort of accident.
Hope all goes well.
