The interviews use STAR type questioning (link below). Multiple permutations and connotations within each question. IMHO, most "real" ATCs are crap with these types of interviews. IMHO, It is not a good 'test' for who will be a good ATC.
Effectively if you feel you did crap at interview, you did, but that doesn't mean you shouldn't be an ATC.
http://www.quintcareers.com/STAR_interviewing.html
Good luck, never a better time to apply, more trainees than ever this year, which means most of the course will fail (the entry bar will be set low, 'ey). Traditionally the pass rate is close to 50% get rated +/- 10%.
Good luck; for the "always apply" folks (I'll get in next time attitude), if you don't get through this time, you seriously need to consider your choice of career; cause it won't be more likely than right now.