I would say yes to the 724.co.uk as Sedo doesn't usually publish sales until completed and it was probably a Chinese number investor with bags of cash.
I would think the investor got a little mixed up maybe as 247 is a better combination, but I'm just guessing really.
I am aware of a NNN.co.uk that sold privately for £xxx,xxx many years ago which I can't divulge... but I've never managed to sell one for more than £100 myself. Had a handful of 0NN.co.uk's a few years back which I sold for between £50 and £100. Unless there's meaning or they lend themselves to being naturally brandable I don't think they're worth much these days. Without branding, type in traffic on random combinations is going to be rare and there's unlikely to be many potential end users given the memorable combinations are limited / already in use. That said you sometimes see hotels and restaurants named after their street number, so you may be lucky in matching up with a potential buyer. Or perhaps a telephone prefix.