i was going to suggest ringing diawa and sending it off to them as normally tackle manufactuers are very good when it comes to replacing tackle and their products.
Ive just looked and found a spool of diawa tournament monofil that i use for sea fishing, i think i use something different for coarse fishing. and yes 8 quid is'nt a budget line, you can pay more and you can pay a lot less but diawa should be a good quality line so im pretty sure it a manufacturing fault.
Dave