I unplugged my EGR valve and sure enough the car was much smoother, no juddering in slow moving traffic. Plugged it back in and the judder returned, the check engine light remained lit but went out after a few days or 30 miles of normal driving, no need to disconnect the battery. Once the weather improves I'll give the EGR valve a good clean.
Note that my normal driving routine is 10 miles to work each way on mostly dual carriageway, so whilst it's a short trip the car is usually driven at 70mph were conditions allow, so the engine should have plenty of opportunity to breath.
I did try three tankfuls of Shell V Power back to back some months back and it *seemed* smoother, the judder was less noticeable but still present.