Wow, I have never heard of this, but I have not been in NHS. It might be that a qualified person is not on staff to administer the contrast? I really am just thinking because I have never heard of this. I have had an MRI at 23:30 and at 03:00 and I have never experienced your situation.
The contrast is most useful to "light up" active lesions. If you have lesions, they will appear in the images, but active ones will not be highlighted. If you have current symptoms and you need to check for active lesions, call your doctor and ask him about your situation. Better yet, call him any way and let him know what you found out.
I hope it goes well for you.