If you've got your new cover note, you could send it recorded delivery to the seller and ask him to tax it using your cover note.
Had the same problem when I bought a van recently, sent a good copy of the cover note but in the end the seller decided he couldn't tax it as apparently they take the V5 at the same time so they can send it off with new owner details. He wouldn't do this as then the van was in my name before I paid for it, but he was a dealer. A private seller may be a bit more trusting?!
I'd go for the trailer option anyway, even if you get it taxed and drive it home it would be nice having the trailer following if it has any gremlims.
Edit: details...can't remember if its the cover note/policy schedule/insurance certificate you need to post...check first

