Right: I'd imagine that in order to persuade companies (i.e. corporate expenses departments) to accept these receipts, they'd have to be convinced that they were the same as the paper originals.

Indeed - and more importantly, HMRC (the tax office) would need to accept their integrity. But, Nectar is a sufficiently large organisation as to be able to offer such assurances and for corporates to accept their word - backed up by contractuals, I'm sure!