Sole,
If you are an instructor, you log the time as P1, the student logs the hours as dual. Also in most JAA appoved log books there is a seperate column labelled instructor to enable you to keep track of your total time as an instructor.
Yes instructing you can build the hours, however to unfreeze the ATPL you will need 500 hours in multi crew operations. So in effect any instructional hours flown after achieving a 1000 hours total time will not further your cause with respect to gaining a full ATPL.