get a free alternator check at any half decent garage or even a trade alternator place before you "splash the cash" (even halfords might do a free check?)
Looking at my screen it looks like you are only getting 14.5 volts at 3000 revs but my amd chip may be lying ;)
You can't check an alternator functionally with an ohmmeter without dismantling it as the phase windings effectively bridge the diode pairs, masking the effect of an open circuit diode.
A voltage check is useful, but if you have a whining noise which is non-mechanical in origin, this generally indicates a diode fault. The machine becomes magnetically noisy with a faulty diode bechause the phases are no longer balanced. The diodes can indeed be checked with an ohmmeter, but you must disconnect the stator first.
I have seen a few Valeo machines of this vintage with faulty stators - the windings come out of the slots, are hit by the rotor and then break and/or are grounded. This will be obvious if you dismantle the machine - remember that Valeo stators are not self-centering. This fault also gives magnetic noise due to phase imbalance.