Crystal on 1986 Rolex: Fake or Genuine

[PaulJJ]

I have a 1986 mid size date just. I have always taken it for servicing (albeit irregularly) at Rolex in Bond Street London where I bought it, most recently 5 years ago, and never to any other dealers. I have just given it to the official Rolex service centre in Ho Chi Minh City where I currently reside. They are refusing to service it unless I replace the crystal which, they say, is fake! So either Rolex in Bond Street has cheated me, or the Rolex centre in HCMC is trying to cheat me (or doesn't know what they are doing). Has anyone else had a problem like this? The money for a new crystal will not break the bank, but I am curious to understand what game has or is being played on me. Thanks for any input.