Slates will collect condensation on their underside any time cars get condensation on them - not a problem..

IMO nothing can prevent it and not worth preventing.

Flaking is what takes out the slates in the end. Welsh slates are virtually waterproof so although they get wet from condensation on the underside the moisture doesn’t soak in. Other slates can absorb some moisture, when it freezes that is what causes the flaking.

I don’t worry about ventilation between the slates and the sarking layer. As soon as the sun comes out the slates get warm and moisture is driven away.

Between the insulation and the sarking, follow the regs. I suspect that the gap is over egged and 25mm would be ok, I have seen less without problems, don’t quote me though.

Also, some of the slates ring nicely when tapped with a knuckle? Could that mean the ones that do might not be as bad as they look? Or is there more to it than that?

Many thanks

Re the sound, a slate in great condition will sound like a bathroom tile does when you hit it. I'd expect the ones with the flaking underside are starting to sound more like wood/a thud sound.

Try breaking it over your knee or hit it more than once with a hammer - the worse flaky ones will break apart or the thud sound changes as the layers are coming apart from each other. Good slates will carry on ringing the same, as you hit them.

Other problems also show themselves when you hit a slate, if there is a hairline split soemwhere the slate will have a rattle sound as well as the thud or ring. I've reinstalled flaking slates before but only as a last resort, and I put them near the ridge of the roof so I don't have tens of rows to strip and re-lay if they do start breaking in years to come

If a slate is holed in the wrong place, you can usually re-hole it further in towards the centre, or some small distance away from the existing hole; you're more likely to run into problems if you have to re-hole further up as the lower hole will be closer to the wind drive line or the runoff fan-out line (more on those below) as these parts of the slates get wet from rain during normal operation. Note that those cyan and green lines are just estimates/guides and in severly exposed locations could be very different

Generally if a slate is damaged above the nail holes you may choose to use it if it's in otherwise good condition, but do remember that the area of slate above the holes contributes to the double lapped nature of a slate roof

In the attached diagram (borrowed from fixmyroof and added to) I've added a red dotted outline to describe the outline of the slate with the faint "y" on it. The yellow area is the area of that slate that is above the nail holes, and you can see that it is required to carry water from the gap between the two green lines. The green lines are the "fan out" line and the cyan dotted line is an example wind-drive line
If there were a hole or some damage where the blue X is, wind driven rain could easily leak there. If the slate were re-holed or had old holes too close to the fan out or drive lines, they could let through (and then they have their own fan-out)

..so be cautious, especially if youre installing a slate with a missing bottom corner etc above a slate with damage above the nail holes - you might have to maintain awareness of defects in what's already laid and defects in the slate youre installing as you go

Oh, there wa some more thing I forgot. On the most recent slating job I did, at least, I found that some of them had a white stripe running across them - they looked sorta like a mint imperial. A few of them broke exactly along the centre line of that white stripe while being handled even though they sounded good. If you have any like that, perhaps set them aside and make close inspection that there isn't a fault in the slate