Interestingly if you try 04301762 (Test Limited!) it comes up with.... a premises when you do a CompanySearch call, but doesn't when you do a CompanyProfile call or RegisteredOffice call.
I had a poke around on a company we've formed and based on that it looks like the CompanySearch returns exactly what we sent through (via XML) on incorporation, which included an (optional) premise tag, but the CompanyProfile has moved Premise and Street to Address1 and Address2 in the api, so there must be some form of conversion going on between the two systems. Based on this I'd be wary of using CompanySearch to process addresses.
Just a heads up that I'm just a user of the APIl... I'd like to see the documentation updated as well!