An attachment of earnings order can't be made unless:
(a) a CCJ has been issued ;
(b) the debtor has defaulted on the CCJ ; and
(c) the creditor has then gone back to court to seek the order.
Given that all that takes time, you'll now be beyond the 30 day period where you can get a CCJ removed simply by paying the money owed, it would seem that you're now stuck with the CCJ on your on your credit record at the Register of Judgements, Orders and Fines for 6 years from the date that the CCJ was awarded.
However if you can show the court that you provided the creditor with your new address (and that they failed to take note of that fact when pursuing you for the debt) you can apply for the judgement to be set aside. If that happens the CCJ will be removed from your record.
If you can't get the judgement set aside you can apply for a Certificate of Satisfaction to show that you've paid the debt in full. (The CCJ wil remain on your record for 6 years but be marked as 'satisfied').
Start by paying £4 here to check that the CCJ actually exists:
https://www.trustonline.org.uk/search-yourself
Then read this and act to either get the judgement set aside or to have it marked as 'satisfied':
https://www.trustonline.org.uk/understand-judgments-fines/ccjs-and-county-courts/removing-a-ccj
Also, first thing tomorrow morning contact the landlord (or their letting agent) to explain the situation. (If possible, actually show them the letter which you've received, so that they can see from the date that you've only just become aware of the problem). They'll be far more likely to overlook the presence of a CCJ on your record if you've explained the circumstances in advance, rather than if you seem to be offering excuses later on.