I believe your problem lies in the calibration of the steering lock.
As you know, the steering lock is actually remote from the key and is operated by a solenoid.
Steering lock calibration is generally knocked off kilter following a low battery voltage or, alternatively, by somebody playing with the diagnostics who doesn't know what they are doing, crashing out of the diagnostics rather than closing it down in good order. They have also been known to lose calibration for no discernible reason other than perversity.
The calibration is a simple five minute task but unfortunately requires the SDD diagnostics and somebody who knows what they are doing. I believe your only option is to get a local indie or dealership to reset it for you. I suspect that a dealership wouldn't do the job in situ and you'd be required to have the car recovered to their premises. Maybe an indie would be more amenable to attempt the reset in situ. Dealerships usually charge 30 minutes labour for the reset using the diagnostics which would cost between £60 and £95, indy would charge less I guess.
Testing with a generic OBD code checker such as those carried by AA and RAC patrols, will tell you only that there is a fault with the steering lock.
Worst case scenario is that the steering lock module is U/S and will require a replacement. That will be several hundred pounds for the module if I recall the price correctly. The job, which includes reset of course, is reasonably straightforward and shouldn't cost more than 1 hour labour charge. Although, having said that, the steering lock assembly can sometimes be a devil to separate from the steering column assembly. Another method is to replace the whole steering column with a used part from a breakers which, as you might expect, obviates the need to remove the steering lock assembly although it is a much more involved job overall and still requires the use of SDD to link the steering lock module to the car's security system.
From experience, for what it's worth, I've seen this fault several times now and each time it has merely required the reset.
This is a good example of why people buy the diagnostics hardware and software. Dead simple and infinitely cheaper to solve the problem yourself rather than calling in the professionals.
If I was closer I'd do the reset for you. However, maybe a JoC member who has the appropriate kit and ability might read this and be able to assist.
I wish you luck and that the steering lock module merely requires a reset.