Thanks for the comments @JeffH and @Semaphore.
We were extremely surprised to find no nectar/honey stored at all. With most of the end of season removals we do there is usually at least a full Langstroth frame equivalent of stored honey, this colony had nothing.
The colony was very strong (in numbers) so we can only conclude they were either living off available nectar/forage or robbing from elsewhere.
Unfortunately, we have local 'bee advocates' which become argumentative when we mention a lot of the hives we find would likely die over winter if not recovered. I think this attitude was summarised really well on the Scientific Beekeeping website with this post:
See 'Beekeeper Taliban'.