The best thing to do is for your sons to write to the credit reference agencies who will put a Notice of Disassociation on their credit files. The two main agencies are called Experian and Equifax. You can find their addresses on their websites. (Perhaps www.equifax.co.uk or www.experian.co.uk)
It may be worth while your sons applying for a copy of their credit reports before requesting the disassociation. THis can be done online for around �10 or by post for around �2. It's money well spent, and quite an interesting read to see what banks can find out about you!