after many more hours of trying and reading I dropped systemrescueCD (www.sysresccd.org
). It comes with LVM commands but lacks the opportunity to open the LV to acces the files.
I started the ubuntu 7.10 CD in rescue mode with bash prompt on /dev/volgrp/vol and suddenly I was able to access this file system.
This seems to be the key advantage of LVM, to have proper access to the files, even if a hardware defect blocks vital functions.
I found the very big file and deleted it, filespace consumption was back to as-usual, however even doing that booting of system was still not possible.
I decided, because files are perfectly readable, to save data on 16GB USB stick and make a fresh setup of ubuntu (this time 8.04) on a new Harddisk.
This is a bit time consuming, I just calculated data transfer will take in the range of 15hrs.