Finally today I had a look at the slight knock coming up the steering column. After half an hour upside down in the footwell, and experimenting with loosening nuts and bolts, I found the source. The hole through the bulkhead is a shade too low and the column is only a few thou clear as it passes through. So flexing means that the column touches the bulkhead occasionally. Luckily the column has two flat sections, big enough to allow a needle file in, so I spent half an hour or so opening up the gap. It's now OK. Here's the gap - "before".................