I live in a first floor flat with a flat above and below is a shop,after 4pm the exit leads to a locked yard that is used to store scap washing machines and parts, how wide has my only escape route got to be?

The width of escape routes and exits depends on the number of persons needing to use them. In your situation I cannot see more than 60 requiring to escape, therefore 750mm width should be considered adequate and it is very important that you are able to open the locked gate to get out of the yard.