It appears that Bamboo branch plans cannot have different requirements to the master plan and this is how we specify which agent the plan is to be run by.
So a Branch Plan will always run on the same agents as the Master Plan.
To get around this I will need to create a new plan for the 2.1 branch on which I can change the requirements and remove the existing branch plan.
Can I get permission to add Plans to Bamboo please ?

The disadvantage of this is the settings of the main branch plan won't be inherited, but new plans can be created based on the main branch plan.
These are not such big disadvantages and I don't think there is a way around the agent requirements issue.