In RMC (Revived Middle Cornish) the statement "you're not going to ... " is introduced with ‹ny›, thus ‹ny wre'ta›. The apostrophe is optional since the 2014 SWF (Standard Written Form) Review, so both ‹wre'ta› and ‹wreta› are correct. So, it's up to personal preference whether you wish to apostrophise. ‹Na wre'ta› would be used in a relative sentence, "He told me, that you weren't going to ... " ‹Ev a leveris dhymm na wre'ta ... ›.
In the RLC (Revived Late Cornish) variety the distinction between ‹ny› and ‹na› is lost and ‹na› is used in both circumstances. You could see that as a "colloquial" form.