Just run Sophos as Euan suggested and it returned up this: Troj/JSRedir-BV in a file named YouTube Message,html [not a comma].
I cleaned it up following the very clear instructions.
Do I now need to run a check on the Time Machine files? If so which ones I wonder.
Well, Sophos must have gone on searching and a message has popped up saying that it has found the same trojan in the Time Machine backup. This time it says I have to remove it manually.
The path to it is: Vol/Ext WD TimeMachine 550GB/Backup.backup/Mark Ford's iMac/2012-06-01-0956/Lion/Users/Mark/Library/Mail Downloads/YouTube Message.html
Trouble is I can't find it!
Searching in the Finder I loose the trail after /Mark/
there it starts being different. /Library/Mail Downloads/YouTube Message.html are nowhere to be seen.
I'm stumped, can anyone offer a way forward please?
This must surely reflect the fact that the User Library folder is invisible in Lion. You will have to try one of the tricks shown in http://www.macworld.co.uk/macsoftware/news/?newsid=3293365 in order to see this folder, and then find the file which you wish to remove.
Thanks Steve, I t does seem likely. I am busy backing up everything I can see to everywhere I have available because my machine is behaving badly (unrelated to this issue I think). When that is done I will see what I can find & report..