Does any one know weather it is legal to work a eight hour night shift on your own and locked in the building with no key the only way out is fire exit
Yes it is legal to work an 8 hour night shift on your own. I can't give you an informed answer about your other point.
However I'd be surprised if a fire exit can serve as a sole means of escape. It's there as a last resort.
There are lots of conditions regarding night work and I'd advise you to check the Department for Work and Pensions website