I respectfully disagree with Nero. The rack has no effect on the caster so they can be checked independently. If you check the caster and it's out, either the front suspension was put together incorrectly or you have bent pieces. In either case, you want to fix that first. I do agree with him about getting your rack right. Part of that is to pay attention to the resistance called for when turning the pinion and moving the tie rods. Less resistance is NOT better.
In my experience the most obvious indication of a bent rack is that there will be a "place" or "places" where it is a little harder to turn through. That is not at all to say that there aren't other symptoms (and if there are others, I suspect JB would know them!). If that is the case, try to find a good used Triumph Spitfire rack and swap the good parts from the Spitfire rack to your Europa rack. The Europa rack has an extension for the tube and a spacer to reduce the amount of steering that is possible. The sleeve nuts on the Europa rack are much longer than on the Spitfire rack (check the S1/S2 Workshop Manual). The tie rods are longer on the Europa rack but I think either could be used (measuring how much of your tie rods are screwed into your tie rod ends should tell you if they can indeed be swapped). Everything else is common to both racks.
I want to apologize for a mistake in my previous post. The shims I believe you are talking about adjust the pressure of the plunger or "shoe" against the rack on the other side of the pinion. It is not associated with the pinion lash. This is the what Nero was talking about. The manual gives two different ways of determining the proper shim pack. One way is to measure the gap between the cap and the rack body and stack shims related to that dimension (again, check the manual). The other is to adjust the resistance to turning the pinion by adding or deleting shims. These two methods do not necessarily give the same result. I would advise using the shim pack that gives the closest to recommended resistance with a shim stack closest to the recommended height. But I contend that if this is properly adjusted, it will have no effect on your steering wheel centering after a turn though it may have some effect on stability at speed.
edit: was a bit more specific about the similarities between the Europa and Spitfire racks.
I lust after the trunnionless uprights that JB has and they would be a big improvement over the stock front uprights but I don't think they will solve this problem.