Thanks all -
To clarify, the 50mm 1.8 is a really good lens but generally not for buses, the lack of zoom is it's main downfall. I could crop, but I don't like to do that.
Since posting this thread I've come to two conclusions, the day time bus shots (see signature for a sample) are OK, it's my group (people) shots which seem not to be as sharp as they could be. That might be an me issue, I fully accept that. I do think though, a zoom with lower than f.3.5 (or whatever my camera uses) would be a good idea generally. Thanks all.