Finish dates move along with Data Date

Hi all,

I'm sure this is a basic problem, but I'm going nuts here. 

I've got a basic level 1 schedule. No logic or anything, just start constraints and durations.

A couple of activities have already started (and the Started check box has been checked accordingly).

Problem: when I change the Data Date the finish date changes (when I wanted it to remain the same). E.g. Activity start date 1st of July, data date 5th of July, finish date 20th of July, 7 day working calendar. Then when I re-schedule witha new data date of 8th of July, finish date now changes to 23rd of July.

Note: Duration type: "Fixed Duration & Units". % Completion Type: "Physical".

I've tried various Duration and completion types combinations, but no success.

Please help! Many thanks.