Turns out the problem I had was that I hadn't correctly added the rootdelay argument.
What I had incorrectly done was just add 'setenv rootdelay 10' to the environment variables. When I checked with the command that Birdman gave I could see that it hadn't been added correctly.
Just for completeness of the thread what I should have done is:
1. On boot up press a key to break into the u-boot Marvell prompt
2. Run the printenv command and take note of x_bootargs_root line.
3. Edit the root arguments:
editenv x_bootargs_rootthen enter what was on the line from point 2 (after =) and add rootdelay=10 to the end of the line.
4. Run printenv again and confirm the only change is the 'rootdelay=10'.
5. Save the changes
Thanks for your help cjm and Birdman.