Thanks all. The tape has been there for 2 years without issue. I can back the cap off a few turns and it still backs up eventually. Both the return line and the vent are clear with no kinks or dips. Ive read somewhere that other probs can cause a sudden increase in return flow but maybe clutching at straws. Very frustrating. I have an engineer onboard tomorrow who can hopefully shed some light on the issue. I will post the solution .fingers crossed. Thanks again.

Hi 5knots

Just wondering if your engineer had any luck sorting the breather issue today

Regards Don

Thanks don and all who replied. My mate who is an engineer came onboard for a look. After querying and suggesting much of what has been said here we were still no further advanced. However he did think the only thing left after eliminating the above was the thread tape. Previously the tape may not have been a good seal and therefore allowed some breathing. It doesnt need to be much. Over time diesel has gotten around the thread and tape and now makes an airtight seal. Its not until the cap is backed off many turns that the tank breathes finally. This would support the idea. Either way ive tapped a new breather into the filler cap how it should be done. I guess i could always remove the tape and replace with some fresh to test the theory but for my sanity i wont go thereCheers all