Thanks lorax, I have spent an hour or so on chat with my hosting technical guys and he wasn't able to resolve it and asked me to submit a support ticket and they will look into it further. The problem I have previously encountered is that the hosting company say they don't support other software but where I am supposed to find an answer is not easy. I have posted in Wordpress forum with no reply. The custom links HAVE worked for a long time and I have hundreds of posts with custom links generated and I get a fair amount of search engine traffic to them. To lose that traffic in favour of the default permalink is painful. AT the moment I have left the custom link in place BUT the older entries link doesn't work..