Not sure if this is technically correct but I thought I'd record this in case someone could a) shed some light on this and b) make sure I don't end up with the same issue in the future.
I had mailscanner setup and ticking over nicely when I opted to enable account specific settings and start editing specific accounts. As I went I realised that noticed difficulties in trying to get various rules to actually retain the settings I was giving them.
It seemed that every time I edited the spamhigh.action.rules I would find that the settings I'd set would revert back and I couldn't be 100% sure it wasn't a browser cache issue but eventually I guessed there may be a problem so I tried forcing and update of mailscanner via the MSFE (mailscanner front end)
...bad idea!
Everything went completely to hell in a handbasket with emails suddenly dropping into a sort of no mans land.
Exim started complaining about being unable to lock or write to the spool and whilst the mailwatch daemon was logging the emails they weren't actually getting into the mail boxes.
In the end I figured the smart move was to contact support for my server and they uninstalled mailscanner and got me back up and running with spam assassin and ClamAV.
Since then I've found a few posts that seem to indicate what might have happened and I have a hunch that when I forced an update of MailScanner it didn't manage to stop the existing mailscanner service and I ended up with two running at once.. One possibly locked up the system while the other went through the motions of recording the mailwatch stuff and borking..
I may never know... however in future before I ever do a MailScanner upgrade in future I will be stopping the Mailscanner service and running this little lot here:
http://www.configserver.com/techfaq/index.php?faqid=30
Hopefully that won't result in a repeat...
Unfortunately I had to kill the original db for mailwatch when I re-installed MSFE as that locked up and refused to update anymore but beyond that it was a most unpleasant experience.
I dunno if this sounds at all familiar but if anyone has any insights I'm all ears... I'd like to avoid problems in future if at all possible.