Anyone know how the steering lock works?

[Enoch]

Thanks Adrian, I see what is happening now. Where the lock barrel goes in to the housing there is a sprung detent ball. As the key is withdrawn the pressure of the detent ball against a tang on the lock causes the rotating part of the inner lock to be pulled out. That causes the metal locking piece to engage with the steering wheel. My lock barrel is knackered so it does not pull the plate outwards as the key is withdrawn. So it looks like a new barrel will fix it.
Thanks for your response, it helped me look in the right place.
Enoch.