Hello amino
I have also got a claim going against 'GE Capital - House of Fraser' & Debenhams

. I luckily had my Debs account number but like yourself once I had cleared the accounts I shredded the cards and everything.
However - I found out my House of Fraser account number by firstly calling a Frasers store and asking for the number of GE Capital. I then called GE Capital saying I had 'forgotten my account number could you please help?'

I cleared security and she offered up the old account number no problem.
I was very surprised she was able to give me this information so quickly and that nothing flagged up as both my Fraser & Debs accounts were passed to Viking for collection about a year ago and I cleared the balance off around July this year.
Maybe it's worth giving them a call to see if you can get your account number that way as well!
Good luck