Samsung ML-4500 mono laser all lights flashing

I've had this Samsung mono laser from new (about 7 years!) - and its
worked great in all that time. But today all the lights started
flashing and it refuses to print. I don't think its the toner cart (I
did try another one that was part used and it still won't work). I
doubt its worth the expense of buying a fresh toner cart to test
(about £40) so I assume its going to the tip.... unless anyone has any
ideas how it can be fixed.

Thanks, Roy