Database Software

I'm setting up rman backups for our test & dev databases. These are cloned manually using o/s commands. As all these databases are clones of prod, they have the same db_id. So I have discovered that I need to use the 'nid' utility to change the db_id in order to register them in the rman catalog. However, if I create a new clone from prod and give it a new db_id, the old backups for the previous db_id (i.e. the previous clone which this has replaced) are still in the recovery catalog.

I can see this info in the recovery catalog views, i.e.

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select * from rc_database
DB_KEY DBINC_KEY DBID NAME RESETLOGS_CHANGE# RESETLOGS_TIME
1 701 3620254146 HOUTEST 3309633197328 21/04/2009 13:14:05
341 342 3688071086 RMANTEST 1 22/12/2008 11:40:30
919 920 3699186203 RMANTEST 3209104 27/04/2009 15:19:52
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In this example, db_id 3688071086 no longer exists and I am unable to report on it using rman commands.

So the problem is, how can I remove these old obsolete backups? I have tried setting the recovery window to a low value and running 'delete obsolete', but as far as rman is concerned, it won't clear any records for the old db_id, and this no longer exists.

At the moment we are just setting up rman on our test systems, which is why it isn't in prod yet.
Obviously I'm missing something fundamental here, perhaps somebody could point me in the right direction?

The platform is Sun Solaris 10, with Oracle 10.2.0.3

Thanks