And so that was it, it couldn't have been simpler.
The man in seat 61 recommended buying my tickets from either thetrainline or raileurope and I chose the latter. They were happy enough to accept a British Senior Railcard for the whole journey and included the cost of a couchette for the night trains. Change from £245 for the whole shebang including booking fees (commission).
I have actually been allocated seat 61 for the first leg of the journey – whoop!