Landlord insurance - building materials cover

I am in the process of refurbishing my penthouse apartment in Nottingham.  Progress is slow as any landlord will know and I'm wondering whether all the expensive building materials will be covered by the insurance policy. 

Are expensive building materials covered by my landlord building insurance?

I'm dealing with a gang of retired or part-time tradesmen.  this means that don't want to work too many hours.  So as a result I have a lot of expensive building materials lying about on site waiting to be installed.  This includes:

• An expensive Italian spiral staircase
• £4000 with of European Oak 
• A partly installed Ikea kitchen and appliances
• Parts of a luxury bathroom

An incident last week where 'somebody' left a window open made me think what would happen if there was a break in and somebody had the where for all to remove all these expensive building materials and would my insurance cover me for the loss?  For a start the fact that the window was left open would probably invalidate my building insurance because there would potentially be no forced sign of entry.  "I wasn't best pleased!"

The law on landlord building insurance covering a development project

As landlord property insurance is not my strong point I thought I'd consult the experts at Alan Boswell to see what their view was on it all.   They admitted that the area is a grey area but in general terms:

a)     If you are living in the property whilst it is renovated, the building materials would get classed as contents and be covered
b)    If no one is living in the property overnight (generally definition of unoccupied) then theft of unfixed building materials tends to be excluded
c)     If you have a contractor in to do the work they can generally cover the materials under their insurance until the work is complete, but you would need to ensure this is specified in the contracts between you as the contractor may have an extra charge to do this.

Looks like the safest bet is to keep the site fully secure and protect it from a break in as a risk to the development project is site theft and the left of expensive building materials unless they are fully installed.

Is it possible to obtain 5 star building insurance?