Originally posted by netbar
Problem is I don't have a backup. (Awful practice I know but this was supposed to be a piddle test db, not so know thought the heat is on.
I hope you learnt a lesson here.

Incidentally I have offlined the datafile in question and then tried to start db is this OK?
I don't think you can just offline and start the db. You may have to do a offline drop, remember you will lose all the data in the file and if that is the only datafile for the tablespace, drop and recreate the ts.