>>>never heard of that grade
Neither has the Universities and Colleges Admissions Service (UCAS)!
http://www.ucas.com/how-it-all-works/explore-your-options/entry-requirements/tariff-tables
(A-level grades are listed in the 3rd column, with the corresponding number of UCAS points - for university admission - in the 5th column).
'D' may well stand for 'Distinction' but it won't be at A-level. For example, BTEC grades are P (pass), M (merit), D (distinction) and D*. The award of a D* gains 140 UCAS points and is therefore equivalent (for university admission purposes) to an A* at A-level.
Chris
PS: I'm reminded here of an ex-pupil of mine whom I met, during the summer holidays, leaving the school premises and clutching his CSE results. He proudly showed me his 'A' grade in mathematics. I'm still not sure whether I should have reminded him (as I did) or kept quiet about the fact that CSE grades used numbers and not letters. The 'A' stood for 'Absent' because he'd failed to turn up for both exams!
;-)