Do not follow the suggestion of A320 rider! If you get a conviction for low flying it WILL affect any future job applications in the industry. In addition, I think it may count as a criminal conviction (check with someone legal about this). If that is so then it could affect air side pass applications and certainly some non-aviation things (ie/if you ever wanted a firearm or shotgun certificate).
To clarify my previous post, I don't mean that you literally get aggressive and rude with the CAA. I've seen this tactic used (sucessfully) by a PPL holder but as you hold a professional licence you have a lot to lose. I would suggest you politely but firmly state the facts (ie/you were not below 500') and that you will firmly defend that position. Make it clear that you will defend you position legally if necessary. I would also ask them what the 'evidence' it is that they have-if your account is correct then it will only be reports by members of the public who we all know won't be able to tell the difference between 400' and 900'.
Not sure who is in the legal department of the CAA these days but good luck!