Ok, back from holiday and I have a promising link.
First of all, I was assuming from the get-go that the purpose of this thread was to mount the encrypted partition so you could get access to the files and copy them to safety, hence my talking about backing them up.
This link
LVM2 - Knoppix Documentation Wiki has some steps on mounting LVM2 from knoppix, so it looks like the steps are:
1. Do the cryptsetup step from earlier in this thread to decrypt the partition.
2. Follow the steps on that link (updated 3rd of may 2008 in case someone evil edits the wiki) to scan for and mount the LVM2 partitions.
3. Let us know how you get on,
Chris...