Lizzie ,
Assuming you can move the bird box without too much trouble , have you seen this ?
Wear protective clothing, especially gloves.
Take a note of where the nest is and how you will reach it when it is dark.
Wait until all or most of the workers have returned – this is often well after dusk.
When activity quietens down, block up the entrance hole with flexible foam (e.g. from a sponge or scouring pad).
Seal up any holes you find around the box using tape, as bumblebees can easily use these to escape from the box when it is being moved.
Take the box down, without tipping it over, and k