Thanks chaps.
Here's their process:
https://www.nulukwheels.co.uk/the-process-2/
The rep says that the pinholes are down to deep-set corrosion that was present in two of the alloys (I'm not sure if some or indeed all of the wheels had previously been refurbed or not).
Wet paint finish (Silver Sparkle) is to be applied over the primer powdercoat. Hopefully this, coupled with some of the repair works already undertaken, will result in a good overall finish.
Will definitely have to keep the receipt for this one, though - in case things start to go awry (cost is very much at the upper end). However, must be said that the company have been very forthright and helpful thus far.
Cheers,
Karl.