Today Judy and Ruth were in and agreed that we needed to find out what the smell was. I opened the filing cabinets (no mice nests! bah!) while Ruth wondered if the dead wasps we'd been finding on the floor might be a factor. Six dead wasps? It seemed unlikely.
Then we looked up.
All the light fixtures were full of dead wasps and warmed by the fluorescent lights, they were rotting. The highest concentration (presumably just below the nest) was above the filing cabinets. Always, always look up. The cleaners were in there with a sacrificial hoover and heavy-duty disinfectant as I left. The smell was... very original.
So, sorry, mr_snips, it wasn't a mouse-nest, and no, TimScience, it wasn't robo-hamsters plotting our downfall, either. Some smug mister went for a suppurating wasp nest, who was it? oxfordhacker? truecatechresis? Whoever it was, I owe you 16p. And a much more detailed description.