Thanks for all the comments guys - after they applied CU6 a couple of days ago I'm seeing PLE consistently at 1.5 hours, the buffer pool is no longer using only 20GB, it's taking 51GB with the SQL Server free memory counter down to 256MB. Much happier with that than I was with what I was seeing before and that obviously answers my question on why SQL Server was leaving such a large amount of free memory.

To address the other points raised - max server memory is set at 60GB - 4GB should be enough IMO for the OS and other services. There is literally nothing else running on the server - any non-essential windows services are disabled, no additional software is installed, there is no virus scanner, things like ole automation, extended procs, linked servers are not used at all. CLR usage is minimal and restricted to admin jobs.

In any event I take on board your suggestion of reducing the max setting and will monitor whether or not the server is being starved of memory for other processes and will consider reducing that if it is the case.

Thanks again for the comments.