One can buy a new brush bar for £27.50, but the Dyson website does not tell you that this is the 4 prong old version. If you want a 3 prong brush bar, your have to buy a complete Direct Drive Cleaner head which contains a 3 prong brush bar, but it will cost £60. Rather expensive when all that is required are the brushes to be replaced.
My solution was to cut a notch in the 4 prong brush bar just above the brush so that the brush itself can easily slide out. A similar notch on the existing 3 prong brush bar enables the old brush to be removed and replaced with the new one with a touch of glue at the notch to hold the brush in place. This has worked very well for some time now and avoids paying for a complete direct drive cleaner.