You will need to remove the keyboard. Usually 2 screws from the inside. Next you need to remove the key. Remove the return springs. Then you will need to slide out the keywire. Usually from the bottom of the keyboard. Now the keys will be loose in the slots but will not come out because the button is bigger than the slot. So you will need to remove the button. Remove the key and turn it so the back side is up mount it in your drill press then use a drill blank or small punch to push the broken flapper wire stub out. If it does not come out with moderate pressure then try heating the wire with a hot soldering iron. the heat should break down the glue (likely it's epoxy). If you are sucessful in removing the old wire then the new wire needs to be pressed in using epoxy as a glue. Reassemble the instrument, install the flapper and bend it into position and you are set. Keep in mind that all flapper wires are made by the individual maker of each instrument and are not necessarily interchangeable.
If this is too complicated and you don't want to tackle it see the above link for a builder in England
Thanks for the help. I spoke to a friend who is a concertina maker and he suggested the soldering iron technique - I managed to get the wire out without taking the fingerboard to pieces. I have made the replacement and friction fitted it. I will now reinstall it with epoxy as you suggest. Thanks again.