Need help cleaning up PostgreSQL installs on Ubuntu 12.10

I'll start by saying this was my error and what I would now like to do
is 'clean out' remnant traces of past Postgresql installs v 8.4 and 9.1
from my Ubuntu Server 12.10 and then reinstall a working 8.4 or 9.1 (8.4
might let me recover and backup an old database then upgrade to 9.1).

I upgraded from Ubuntu 12.04 to 12.10 and it seems to have added
postgresql 9.1 while leaving the previous 8.4 install and these
conflicted making my PHP app unable to access it's database. I tried
removing one, then both and got to my current mess.

What I get at present is:
sudo apt-get purge postgresql

E: The package postgresql-8.4 needs to be reinstalled, but I can't find
an archive for it.

sudo apt-get purge postgresql-8.4

E: The package postgresql-8.4 needs to be reinstalled, but I can't find
an archive for it.

So then I try:

sudo apt-get install postgresql

E: The package postgresql-8.4 needs to be reinstalled, but I can't find
an archive for it.

trying a bin file:
sudo .postgresql-8.4.14-1-linux.bin

Error: A service A service (postgresql-8.4) already exists in the
'/etc/init.d' directory

Before I do more damage can someone give me an idiots guide to cleaning
this up so that as a new tax year starts I can restart using the PHP app
that requires Postgresql.

Thanks
--

*/Mick/*