I think that serving SARNs on Experian and Equifax is a very good idea.(posted 8411 days ago)Their addresses are:
Equifax
Dept IE
PO Box 3001
Glasgow G81 2DT
Experian
Talbot House
Talbot Street
Nottingham NG1 5HF
Address the letters to the Data Protection Act Compliance Officer, follow the advice in the posts above so that you know what to ask for, and enclose a ten pound postal order per SARN. It's also best to send a SARN by Recorded Delivery so that you have an independent record of when the 40 day compliance period starts.
Good luck.