Just adding a conclusion to this thread as it really winds me up when i read about a problem and the tried solution has not been confirmed as working!
I purchased the 100UH, 6.1A Inductor below.
It was installed on the same input as the fuse. I chose the one above as it has a plastic bas that I have araldited to the top of the converter.
My clamp meter arrived, with a near flat 12v accssory battery I am seeing 3-3.5A on the input side and 30-35A on the output side. This Means that the DCDC is taking in much more power than its 400W rating.
My only worry now is heat. I purchased a laser thermometer, after about 30 minutes use the DCDC is around 80C the Inductor is about 70C (but mounted on top of the DCDC.
Heat soon drops once the accessory battery is fully charged.
I tested the solution by flooring the car (smoking tyres etc.), I pulled 600A for a couple of seconds several times. The fuse did not blow!
I also purchased a bridge rectifier to use as a diode but have not needed it.
Just to say a big thankyou to everyone that has helped me solve a very irritating problem that should never of existed!
Any thoughts on the heat issues?