Cheers folks (at least means people are reading it!

)
This is the second time in as many weeks. First time a full delete and re-upload cleared it up, this time re-uploading the main welcome pages appears to have cleared the issue.
I do, however, have a request for info from my webhost because I could really do without this happening on what has become more-or-less a regular basis. In the meantime I'd sadly have to say to take Magnatom's advice and run a virus check first.
I am this

with the webhost.
p.s. And thank you to everyone who tried by whatever means to let me know - the problem didn't show up in Safari and I hadn't checked the site on the work pc for a few days.