Unfortunately the upgrade to Kaffeine didn't help, so I tried a few other things to try and solve my problem. Since nobody else seemed to have the same issue, it must be related to my individual set up. Turning off desktop effects made no difference, however changing back to the default Oxygen from the QtCurve style I was using did the trick. Which was when I realised the problem had been between keyboard and chair all along, I was using a theme using transparency in QtCurve. After disabling window transparency in Kaffeine (SMplayer had this disabled by default, in the theme I was using), everything worked as before. It seems that the upgrade to KDE 4.5.1 coincided with my Kaffeine issue, and I 'forgot' about switching to a transparent QtCurve theme around the same time!