I've only speed read this but I work for Comet so maybe I can help.
For a start, the debt you have will be with GE Money, not Comet directly, they provide the finance services. Depending on when you made the purchase its probably 19.9% APR.
Now the fact that i'm on a Debt management plan with the CCCS (i know) means that I know that the so-called debt collectors you've been passed to are probably 'Viking Debt Collection Services' another department of GE Money , which is basically an indian call centre. Deal with GE directly as they are UK based (the number will be on your statement) and they will advise you best. The debt collectors don't add charges so don't worry, they are just there to get as much back as they can for the company.
Without sounding biased, and having debts with several creditors, GE are actually one of the better ones for helping their customers in situations like this.
As for speaking to Comet, well, I have worked in the stores for nearly six years and I can tell you now no store is going to take that lot back from you and cancel a credit agreement if you walk in there with that story. You have a loan, not a hire purchase agreement, so they won't accept used goods back.
You need to deal with the finance company directly, not Comet.
EDIT
Oxbow said:
This is now serious
If the finance agreement is in your fiances name and address, they will be knocking on her door and either ask for payment or the goods.
Debt collection services are not the same thing as baliffs. This will not happen - don't worry. The worst that will happen is some iffy looking letters (also nothing to worry about) and some phonecalls from people that can't speak english properly. Even if you ignore them for about three months all they do is hand things back to the main GE office to start CCJ proceedings.
In your situation the best thing to do is to make small token payments (maybe 5-10 quid) to keep the account ticking over and like I said, talk to GE Money.