Update 2
Think I've nailed it. Browsing through the C drive with Ztree I
spotted a dll in the system32 directory called Gremyvk.dll. Firstly it
had the read only, system and hidden attributes set which made me
suspicious, the name immediately made me think of gremlin and it
wouldn't let me look inside it. Googled it but no hits so I popped
into the Recovery Console, removed the read only and other attributes
and zapped it anyway. If it was a known useful dll you'd think there'd
have been some hits on Google so I figured it had to be brand new and
potentially nasty.
Result!!! - no more background uploading going on to microsoft-ds,
MBAM updates itself quite happily again, I can get into all the
antivirus websites again with Firefox. Job's a goodun. Funnily enough
I can't get into Bleepingcomputer now to let them know I've fixed it
so either they're coincidentally down at the moment or there's a small
legacy of the nasty's presence still lurking somewhere.
Score another one for the Snark hunter