7. Where, in the case of any action for which a period of limitation is prescribed by this Law-
(a) the right of action is concealed by the fraud of the defendant or his agent or of any person through whom he claims or his agent;
(b) the action is for relief from the consequences of a mistake,
the period of limitation shall not begin to run until the plaintiff has discovered the fraud or mistake, as the case may be, or could, with reasonable diligence, have discovered it:
Provided that nothing in this section shall enable any action to be brought to recover, or enforce any charge against, or set aside any transaction affecting, any property which-
(a) in the case of fraud, has been purchased for valuable consideration by a person who was not a party to the fraud and did not at the time of the purchase know or have reason to believe that any fraud had been committed; or
(b) in the case of mistake, has been purchased for valuable consideration, subsequently to the transaction in which the mistake was made, by a person who did not know or have reason to believe that the mistake had been made.