Cache lookup failed for relation, when trying to DROP TABLE.

Hello,
I've just encountered a problem that I don't know how to deal with.
After having a play with SlonyI, I dropped the entire slony cluster
schema, and then tried to drop a schema which held some slave tables
for the slony cluster.

I kept getting the following error:

ERROR: cache lookup failed for relation 4667548

So, I cleared out everything I could manually from the schema,
leaving four remaining tables that refuse to be dropped, giving
the error above (the relation oid varies though).

Anyone know what this means?

Is there a way to forcefully remove these tables and the schema
using the pg_catalog tables?

Is there any other information I should provide that may help?

Specs:
Redhat Enterprise Linux 3
PostgreSQL 7.4.5
Slony-I 1.0.2

Cheers
--
Mark Gibson <gibsonm |AT| cromwell |DOT| co |DOT| uk>
Web Developer & Database Admin
Cromwell Tools Ltd.
Leicester, England.