Tell em that you've been having really bad headaches and muscular pain, and that you've been dosing up on Neurofen. I've heard (but CANNOT CONFIRM!) that Neurofen can show up positive for Marijuana.
There are alot of medications out there, that could provide a negative result. Try to find out what they are, and say that you've been taking them.

I always carry a half-empty packet of Neurofen in the car etc, in the faint hope of a blag lol

Drug testing is a pile of shit, anyways.
My philosophy is this:
If someone is doing the job properly and safely, and they happen to take drugs...what's the big deal?
If they are taking drugs and endangering others, then that's a different issue all together.

Good luck, Cheech