I made up my own questionnaire, but based the questions on another one because I want to compare the results, but the original one(s) are not really suitable for mine because of the built in bias in the other ones, so I had to go it alone. I suppose it depends what your questions are for, if yours are, as you say, for a major part of your PhD, then it would just be another thing they could pick up at viva. Mine is just for a minor part and is supplemented with other info etc from different sources. If you have reservations, and can point to literature which proves your point, I think you could argue that you don't want to rely on a potentially flawed method - is there any chance you could use it for a minor part though, as a bit of a compromise? perhaps they could find some money for you to 'pilot' the questionnaire for them? - then perhaps you could write it up as a paper for a journal! :-)