The end of the matter (hopefully): I went onto the Malwarebytes forums and posted my problem. I got a reply from a malware removal expert, and over the next couple of days he patiently got me to go through a series of scans with different downloaded software, and these found some remnants of the original 6 items of malware, which were also removed. As well as being annoyed at myself for letting this happen, I'm disappointed that Norton neither stopped the malware from downloading, nor found it when I ran a scan. However, it seems that the major security suites are vulnerable to some clever malware, so I'm sticking with Norton, but have purchased the Premium version of MBAM - this works out at £10 pa for each of our 2 computers, which is a modest amount for the increased security. Thanks again.