I completely sympathise with your situation because it could so easily be myself in the same position... I've been working for a number of years in a similar environment without my past catching up with me, like yourself my conviction is now spent and luckily when I got the job I was never asked about anything because they were desperate (I now know why but that's another story).
I can understand the position they are taking because no one is irreplaceable and the needs of the business will come first.
In a small company there is always the possibility they will see beyond your past but with 500+ workers mob mentality won't be far away. I know it's not what you want to hear but it's only a job at the end if the day... Your first priority needs to be to protect yourself, have you informed your RMO?
The company will probably go down the lines of "they can't guarantee your safety in the workplace" and dismiss you on those grounds (been there myself before I was convicted), what ever terminology they use I would recommend you either take someone with you (as a witness) to any meeting's or record them on your phone etc.
If they let you go you have nothing to lose taking them to the cleaners, legally your conviction is spent and they won't want there company name dragged through the mud in any context, they might offer a severance package but don't let them bully you into quiting just to save them a few quid!
I hope things work out but I'm not that optimistic, I could always be wrong though so try to keep your spirits up... How are you coping?
Living in fear is not a nice place to be, I've turned down two promotions and applying for two other senior positions at work just to keep my head down, and recently I started to think someone from a different department had found out about my past... Needless to say I'm giving them a wide berth, touch wood it's been a few weeks and nothings come of it so fingers crossed!