A bit of an update..
I re-potted the plant into something a bit larger but it continued to degrade.
I've read they don't like to be over-watered however the degradation seemed faster when given less water, so I this seemed like the greater of too evils and I've since made sure that there's always an excess of water in the pot the actual plant pot sits in.
It's well-shaded (as I've read they like) so the only other environmental problem I can think of is nutrition, so it was my plan to try and get some appropriate plant food.
That being said it's since started to sprout some new leaves near the centre of the plant, so I'm wondering if the dying outer leaves are due to some factor they were previously exposed to..? Equally, the stalks that many of them are on are so long that they've kinked under their own weight, which will doubtless hamper the flow of water and nutrients to the leaves... so perhaps this is a factor.
As it stands I'm just trimming off the leaves / stalks at their base as they wither and die, and will look to get some food for it at some point.
I think I'll probably end up at a point where all the original leaves are dead / gone while courting new growth from the middle - which isn't a bad thing considering how much of my table the existing ones had claimed..