I am trying to delete old invoices in previous vat quarters but it is asking me for a password, I enter my log in password but it won't except

Hi luggsgarage

This is probably due to a user, maybe your accountant, activating the close the books function which, in this instance, , this essentially prevents you editing, adding or deleting a transaction before the date entered. 

You can de-activate this function by: selecting the cog/gear icon in the top right hand corner of the screen > account and settings > advanced > in the accounting section (the top section) select Close the books > untick Close the books.