Ok it worked. "fdisk - l" told me that "/dev/hda7" was linux swap type 82 (I already knew that). "swapon /dev/hda7" didn't because the device was busy. So I logged out and in again as a a failsafe x-windows terminal - and voila "mkswap /dev/hda7" works.
Is this really mostly because I restarted my system so that the new fstab refering to the correct hda7 not hda6 was loaded? That's my current guess.
Thanks all for the help, Duncan
