Thanks for your replies - the first post matches what I have been seeing, but contains no login username that I can find. However, your second post made me realise I was looking in the wrong spot, and based on the detail you sent, I have now found this info in the radius accounting log. I didn't know radius was involved in this, since it is only a stand-alone server, and so was only looking for logs created by ppp and pptpd.
Problem solved, thanks for your help!