Worth a try talking to the Fire Brigade, but I don't think they will come out to undertake fire risk assessments any more - not for free, at any rate. This all changed in about 2005 when new fire safety regs came into effect and responsibility was shifted to business owners to undertake FRAs.
For what it is worth, I think Chuck is correct - this is no different from an arrangement that two maisonettes, one above the other, would have. There should be a 30 minute fire protection between the two properties however, which means there should probably be a fire door on the inner downstairs door of the apartment.