Well the neighbour with the boiler in the cupboard under the stairs with the smelly flue has agreed to get it serviced on Tuesday. The smell is always in the stairwell area or in the part of our flat that's directly above the stairwell, so I'm back to thinking it is gas from that boiler. The flue is stinking right now but the smell isn't around inside, so it's definitely an on/off smell even when the boiler is on.

The emergency gas guys tested it twice already and found nothing, although both times they arrived to test it the smell had disappeared. Since the boiler servicing will be during the day while the smell is usually not around, that might not unearth anything either. I suppose all we can do is make sure the neighbour makes the boiler engineer aware of what's been going on...