Really useful comments here BigChad - thanks. In my case the EGR valve was replaced fairly early on but seemed to make very little difference really - an expensive experiment! I was also not doing myself any favours - my mileage has dramatically reduced in the last couple of years so opportunities for a good blast to burn off rubbish were also reduced. Additionally, although the car was always serviced by Lexus (so I assume the correct oil, etc.) I was consistently using supermarket fuel. A casual comment from the Lexus Service Manager suggested, like yourself, that non-supermarket fuel might help so I changed and the difference was staggering - over the course of a couple of weeks the problem disappeared almost completely and continues to improve to the point where, today, it's pretty much completely gone. I had no idea saving a few pennies on fuel would end up costing so much and was convinced fuel was fuel, no matter where it came from - I won't make that mistake again.
Thanks again for your comments and input, and apologies to anyone following this thread for not keeping it up to date.