Thanks for the input guys. After reading your comments, a bit of research via youtube etc and some experimenting I tried again on Saturday with a much higher degree of success.
The biggest problem I found was that the camera was front focusing to a greater or lesser extent with all my lenses, added to the fact that my 300mm also has a tendency to front focus meant that I had to dial in +12 on the micro adjustment to correct it.
I also changed the AF settings to: -
- Initial AF point - manual (centre point) as recommended by Nigel & Wolfie
- AF Characteristics - Case 6 modified as recommended by Arthur Morris in this video https://www.youtube.com/watch?v=BNAk3_Ao6QE at 4:08
- AF pt selection to Zone (15 pt) or Large Zone (25pt)
Although I use back button focus all the time when I'm shooting portraits, I didn't set this up although its probably something I would have had a go at if I'd had more time.
Looking at my DoF app on my phone, shooting with a 300mm lens on a 1.6 crop at f4 at distances sometimes as close at 12 feet & up to 35/40 ft the DoF varies from teeny tiny ~1.5 inches to quite small ~10 inches so it gives an idea of the challenge that the focusing system has to deal with especially with seagulls wheeling all over the place.