You can reboot while holding down the Command-R keys to boot into the Mountain Lion Recovery Partition and from there reintall a fresh copy of Mountain Lion from the Internet for free. From there you can also use its Disk Utility on your Macintosh HD to first "Repair Drive", in case that simply fixes it. And you can also run "Repair Permissions" from there.
Oh, and please Update your system Info in your Forum Profile. It says you are running Tiger 10.4
Hope this helps
Currently Being ModeratedNov 13, 2012 1:04 AM (in response to Radiation Mac)
Thanks for the advice but I nearly had a heart attack when after installing a fresh copy of Mountain Lion, all my files had disappeared. Having calmed down, I reinstalled from my Time machine backup and I now wait to see if mail behaves it self over the coming days
Will update my porofile