Hi,
Whoever the 'credit' card/storecard is with, if you've only had it a month, they've probably not even submitted the 'report' to the credit agencies about you yet. That said, a trail will appear on your Credit Report itself. I'd say call up Experian or Equifax and ask them if or what kind of impact it'll have.
Currently, lenders are looking for those with very good histories for lending decisions, so anything to help that is good. Closing accounts that aren't needed is also considered a good thing as well - on my report they come up as 'settled' - so, it's a good thing that 'status' is recorded after you've successfully borrowed, paid back and then settled the account - it looks good. If by closing this account you mention, you'll have a 'settled' status recorded, then after you speak to the credit reference agencies, perhaps go ahead?
The credit reference agencies also do change the goalposts around a little bit in line with things that are happening on the greater economic view, so speak to them and see what they advise
Cheers, DtM! West London & Slough UK!