When the tracking was done they did this with the wheel straight, despite my insisting that the natural position was 12:05. I'm sure that this mis set the steering box....
Hang on... If the relationship between the steering wheel and the steering box has not been changed, then they were correct to adjust in the centre position, as the steering box will have been in the centre of the damping "zone".
If the car doesn't then drive straight in that position, something else is wrong - either camber difference left-right, the toe was not adjusted correctly (maybe there's a little slack in the steering and the toe moved during adjustment) or some other factor such as tyre problems caused the issue.
Either way, adjusting with the steering box
not central will make matters worse.
If you suspect the steering box not to be central the only solution is to get to the marking where it joins the steering column and check. Then get the geometry properly set with the wheel central; then get some new tyres on.
In fact, I might be inclined to try new tyres first if you're currently on slicks.
Some tyres make an Omega drive horribly when they are getting low on tread.
Kevin