These laser modules are incredibly fragile, and very easy to damage irreversibly. However, in comparison to the cheaper modules, those found in CD-ROM players are built like tanks (see pics).
I pretty much determined right from the start that it wasn't a dust issue from the behaviour of the system -- and apparently, from one of the repair faqs posted (thanks for that), the light emitted isn't affected that much by the amounts of dust that might enter the unit over time. In fact, it is interesting to note that the laser diode isn't actually situated directly under the collimating lens but hidden away under to a side at at 90 degree angle, so that the beam is reflected off an inclined reflective strip through the lens. The lens is raised and lowered using a delicate solenoid based mechanism.
The problem I had was that the diode had failed. I don't know what the rating was for this particular diode, but they seem to be rated at far lower than your typical LED, at around 7 mA, perhaps someone can correct me if I am wrong. I didn't want to risk damaging the good laser by adjusting the trimmer which I imagine is there to limit current, so I "simply" (it wasn't) transplanted the better diode into the circuit ...and it works!
I don't know if anyone else has noticed this, but some CD players use light on the red spectrum and others use blue. Since CDs are made to a certain specification I would imagine that both are equally effective, but that perhaps the photodiode might have to be matched according to the wave~length. However, blue photon's are more energertic than red ones so I was wondering if there would be any advantages to using blue over red....
.o:0|O|0:o.