Hi, I had the same issue two years ago. I resolved by steering gear refurbishing. Any other attempts to fix that, such as related wheel alignment or bushes changing, had not any effect.

sid wrote:

so what was the fault with the rack?

It was diagnosed as "tight spots within the steering rack". Although Warren did say that sometimes the car would go straight when the steering is pointed slightly to the left, then to the right and then in the centre depending on how it felt.