I think some of the above are an awful lot of work and risk for a single colony. Of course this could be the legendary survivor colony whch is resistant to varroa, in which case it is perhaps worth trying something to to get then out first.
If it is part of a flue try and find where the fires were and see what pumping a lot of smoke up the chimney will do. It is just possible you may be able to drive them out - ideally into a nuc placed somewhere near the entrance. A chemical repellant of the type used for clearing supers would probably help. Alternatively, drill a hole in the wall or carefully remove a few bricks (carefully so the rest don't fall on you) where you think the colony might be and blow smoke in that way. I have driven a colony out of the roof of a porch this way.
However, not at this time of the year - wait until April I suggest.
If that doesn't work there has been a method described for getting a colony out of a hole in a wall which uses a sort of trap to prevent bees which have emerged re-entering and directing them to a nuc placed beside the entrance. A cone of greenhouse shading was what was used in the article I saw. The wide end of the cone is stuck over the entrance and the narrow end points outwards. I have heard two desciptions of this method - in one it said the queen will eventually emerge and join the other bees but in the other account it said you have to add a new queen to the collected bees. Which would be a shame if this really is some sort of survivor colony.
If you are going to try this probably best to seal up all the entrances except one with sponge to make it easier to set up the one-way valve. The method is not quick - at least 3 or 4 weeks for a result.