I think jackdaniels' point is that the PSU was described as silent, and wasn't, and that therefore it was either faulty, if it should have been silent and that particular unit wasn't, OR misdescribed if all such units make that level of noise. In either case, he has an argument for rejecting them, because both aspects are part of whether the goods conform to contract or not.
If he chooses to take this to court, and the argument is about description, then it's a subjective assessment of what the description means. The standard definition is what a "reasonable person" would accept, given any descriptions of the product. "SuperSilent" or "ultra silent" is pretty suggestive of the level of noise that "reasonable person" would expect. It's anybody's guess how the court will see that.
If it's about a fault, then you'd have to be able to justify, to the satisfaction of the court, that they're not faulty and it's questionable as to whether an internal technician's word would do it or not. For the first 6 months, the presumption, by law, is that they're faulty unless you can establish otherwise and after that, the burden of proof switches. There are procedures that the court would normally expect to be followed.
One point I would make is that, if I'm reading this correctly, the dispute is about the £10 "fee"? Is it worth pursuing £10 in court? I understand the point of principle ..... but £10?
Also, if it does go to court, one thing the court WILL expect is that both parties have made all reasonable efforts to settle a dispute and that court is effectively the last resort in the case of irreconcilable differences. This seems to have deteriorated into discussion of court cases awfully quickly.
One possibility, of course, that perhaps both parties might consider is to split the difference - £5 each. It's just my opinion, which actually means very little of course, but if this does end up in court over £10 I rather suspect the court's perspective would be to regard neither side as being reasonable, and that it was wasting the court's time.