Hi Clvspencer
Hopefully this will help (bearing in mind that this is only guidance as really we would need to see the premises or a plan to give you a definitive answer:-
In shops with more than one fire exit guidance says that you should be able to reach any one of those fire exits within 45 metres
If you only had one fire exit (ie just one door which acts as the entrance and way out in the shop) then you would have to reach it within 18 metres
So basically you can have a larger travel distance if you have more than one fire exit.
As long as you can reach atleast one exit within 45 metres then it doesnt matter how far you have to travel to an alternative fire exit should the first one be blocked.
Your Health and Safety Officer may well be correct in saying the door to the stairs isnt required for means of escape purposes (cant say for sure without seeing the premises) but if it isnt then it must not be signed up as a fire exit.
Furthermore I think its a bit poor that the Health and Safety Officer didnt explain his or her reasoning to you, but atleast you had presence of mind to question it.
There are guidance documents available to download free which shows you some of the things we have have been talking about regarding your question (then again you may have better things to do that sit and read through boring guidance) Eitherway the link is
http://www.communities.gov.uk/fire/firesafety/firesafetylaw/aboutguides/follow the link to that page then select the "Office and Shops guide" from the right hand menu.