“80.—(1) This regulation applies if it appears to the HMRC that there may be tax payable for a tax year under regulation 67G… or 68 by an employer which has neither been— (a) paid to HMRC, nor (b) certified by HMRC under regulation 75A, 76, 77, 78 or 79. … (2) HMRC may determine the amount of that tax to the best of their judgment, and serve notice of their determination on the employer. ….”