Principally, missing or incorrect SPF records!
You have an advantage over me in that I don't know any of Nige's domains — I presume you do because he sent you emails? Anyway, though I'm no expert in these matters, I'm comparing my own
dedicated server DNS records with those of eUKhost's Viking and Liverpool and, in my case, the SPF records of server itself and now all accounts use the same master IP of the server, whereas Viking and Liverpool do not follow this format.
Whether this is good or bad I'm open to suggestion — all I'm saying is that most recently since aligning these records I have not had any mail held in queue due to deferrals by Yahoo (but it's probably too early to tell).
So yes, RSVEcosse has no SPF records for a couple of his domains I checked and he's on Viking, so I'm left wondering whether the changes that Mark claims to have made have yet to propagate?
Whether these factors will have positive influence on Yahoo servers remains to be seen — you suggest not, likewise eUKhost — is that correct? Whether you confirm or deny I'm going to continue with my current test ...