Two choices: fast and nasty or slow and kindly.
1. Put a few drops of butryric anhydride (Bee Go) on a blob of cotton wool and insert it into the colony, preferably to their rear.  They will rapidly evacuate and form a cluster which you can gather as if a swarm.
2. Adapt their entrance so they have to go through an empty nuc. When they're used to it, add a couple of old combs. Next, put a bee escape over the entrance so the bees that leave can't get back in, but are able to use the nuc. Add a frame of brood to the nuc to keep them attracted and busy.  Add additional frames of brood from time to time.  After about a month all the brood will have emerged from the colony and the queen, attracted by the pheromones of the bees in the nuc, will leave the trunk and join them and start laying again. Remove the bee escape and replace it with queen excluder.  This will enable the bees in the nuc to rob out their stores while preventing the queen from re-entering the trunk.  When the stores have been removed, move the colony several miles away so they don't go back.
I've used both these methods several times successfully, removing colonies from buildings.  Method 1 is more difficult in the UK nowadays as butyric anhydride (Bee Go) is no longer available here as, for safety reasons, they aren't allowed to send it here by plane and they've forgotten how to ship things by ship.
Chris

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