Alex, check the steering universal joints for play. Last year I found the lower one to have a bit of play in the clamp part, found during MOT. Very slight play only, needed a fairly strong pull to notice it. Sorted by tightening the bolt. I missed it during my pre MOT check. I had not noticed the gradual steering effect but did after tightening ( possibly my imagination though). Rechecked a couple of times since and now ok.
