Downgrading using apt-get?

[Steve Simons <steve@kinderscout.plus.com>]

I've made a bit of a booboo recently.  I apt-get installed a recent, 
unstable version of mysql and it pulled in a few other packages (I can't 
for the life of me remember what they were).

The problem is, licq seems to be broken.  I'm running potato, and 
whatever version of licq is stable at present.  When I try to run licq 
now, I get a load of messages complaining about the IniFile and too many 
open files:  

[WRN] IniFile: Warning, failed to find key.
File    = /home/steve/.licq/owner.uin
Section = [user]
Key     = "RCG"
..etc

There's a similar bug been reported (59540) but I can't find any mention 
of it on SourceForge's site.

Anyway, what I want to know is, can I force potato to downgrade to the 
previous version of whatever it was that caused the problem by using 
apt-get upgrade somehow?  I only have potato and helixcode in my 
sources.list again, but upgrade and dist-upgrade report nothing to do :(

Someone point me in the right direction please, I couldn't face a 
reinstall now.

TIA,
Steve.