Thanks for the speedy response before. Much Appreciated

I'm having an issue which i'm hoping doesn't affect my quest.
Basically, as I said in the last post, i'm already a Vampire. Only this time, i've finally found Fort Dawnguard. I've ran past the guard outside, straight to the doors, gone inside, and Isram & his cronies are attacking me, with Isram surrounding himself with a ball that sets me alight. All because i'm a Vampire, and yes, I know they hate Vampires.
Is there a way around it, or do I have to start a new game? I've tried curing myself of all diseases with the potion.
I'd be massively grateful, as it's driving me bonkers, and I really don't want to have felt like i've wasted 1600 points on something I had been waiting for for ages.