Just found out today that if you do your self-assessment tax return on-line the 30 September deadline (non-self-assessment) doesn't apply - in fact the bloke seemed to imply there isn't a deadline although there must be.
online service register/login
You need a User ID which has to come through the post - have lost DH's (I'm his "accountant" - not a good one!) so have to wait for them to send it again (hence the Sept 30 discovery). It's a horrible thing with random letters and numbers so try not to lose yours.