Hi,
in the team server faq you can read that the backup for your data is kept for one year, And that you can get any time a backup of your data by using default subversion tools. Did you try by Tortoise SVN or so?
Beside that Robin, don't understand why you have the need to go back 20 revisions back. If there are situations where you have risky implementations which probably need fallback scenario's, you could also develop in branches, merge to mainline and keep your branche line...which works fine for me.