Without hearing what is going on it may be the ignition barrel has a fault,the key should spring back from the crank position,if not then through time the starter motor would be engaged with the flywheel with the engine running,even for a short space of time it would soon damage the starter itself,it may well have a broken return spring,whether the spring is internally sealed in the barrel itself,sorry couldn`t tell you that but the barrel would need to be removed for a look,if it comes within the barrel itself then you may need a new barrel built up to suit your key and it would probably be a main dealer that you would have to go to with your V5 and some ID to get a new barrel supplied. Good luck.
cdc
April 2016