Thanks. I kind of got the idea anyway, it was more the application of and the principles of working within the software that was confusing me. It's all good information though so thank you.
I think I'm getting the hang of things now - I've found that I don't need a very detailed mesh in the retopo room anyway - if I just trust to 3DCoat getting it about right I can adjust the mesh at the low poly stage in ZBrush and project all to get it to about the right shape. Then, using a combination of the project brush then dividing and projecting as I go, I can 'grow' the mesh into the correct form. My mistake was trying to project hi detail into the new mesh straight away. This new gradual technique seems to give better results.
Thanks for all the help and advice