First of all, don’t worry too much about these threatens, as doesn’t look to me the bank has many chances on going criminally after you. You have not participated in a fraud unless you got something out of it.
Fraud described in the Spanish Criminal Code has to include:
1. An action by you to damage someone else.
2. This damage must give you a profit.
3. You must know what you are doing.
In your case, even if you were paid your services for providing these P60’s, you were not giving them to the bank directly and you did not know what there were being used for.
The bank can sue criminally the clients (as they are the ones that admitted those p60’s were theirs) and the agent (probably the real fraudster, who was providing the banks with false information (knowing this was false) to obtain the mortgages which, of course, gave him a profit (the banks pay high commissions to these agents for obtaining clients).
Therefore, have someone to check your post box every now and then and, if you are called by the Court, just get some legal advice and you should be ok. Although, in my opinion, the bank will not sue you in any case.