The reason why you'll have been sent a letter threatening debt collectors from GE Capital for being in arrears by #15, is because of their methods of risk management. At the time you were a student, and to them that means you were a "high risk", nothing personal, just that their research shows that students often get into arrears or don't pay, people with a history of late or non payment would be considered high risk, or even those from a certain postcode area. So when you went into arrears, your "high risk" status generated a nasty letter. If at the time you had been in a different employment that was considered a "low risk", then your #15 arrears would have generated a letter saying something like "we're just reminding you you're late with your payment, please send it as soon as possible, we're awfully sorry to trouble you".(posted 8712 days ago)If you were in arrears because you had missed a month's payment, then this would show in your credit record as something like 000100000000 meaning that out of 12 months, in that 4th month you were late, you'll have seen this fully explained in the leaflet that Experian sent you.
As explained in the other answer, credit scoring is a strange game, and there is no hard and fast rule to say whether or not you will get granted credit. Depending on when the arrears happened to when you applied for new credit could have had a bearing on the decision if they were close together. For information, if it was another store card you applied for, GE Capital finance 90% of the storecards available, as well personal loans and mortgages through some companies, so they would check with their internal systems if you already had credit with them.
You'll probably find that when you apply for a mortgage, that missing a payment for a store card won't go against you, the lender will be interested in your payment history now and for the future now you're working, not when you were a student.
best wishes
Pendle