After many, many hours of trying to get the 80m filter working as described I finally gave up and re-read the article.
I learnt that the 80m filter is a two pole filter so it can cover both 80m and 75m, that is from 3.5 MHz to 4.0 MHz. Of course, here in VK we only use 3.5 MHz to 3.8 MHz so perhaps I don’t need the second pole of the filter.
I took a good look at the 40m and 20m filters and adopted similar design for a new 80m filter.
I picked some new values and calculated some new coil sizes. I was about to wind some new coils but decided I would have a play with the existing coils.
I changed the configuration and capacitor values and got just over 1dB insertion loss between 3.500 and 3.800 MHz.
I may try and get it even better with some different coils but not now as I want to get on with the 160m filter and see if I can get that completed before this weekend.