Well, having worked with engineers/scientists and techs at two of the biggest fuel retailers in the UK, I understand the cheapo stuff has less of the good engine-cleaning agents in it.

Furthermore, the same people agreed there was no need to be filling your tank with the expensive Ultimate / V-Power options each time either (unless you want the claimed fuel-saving benefits). If its a decent car, one tank in ever four should suffice to ensure you benefit from the additional treatments the higher quality fuel has in it to look after your engine.