Subsidence warning after building survey (SE Lonodon) + high premium on building insurance

SKchloe

Dear wise people in this forum,
We are facing frustrations and hopes in the process of property purchase in South East London. (SE4 near SE15)
Like other many people in this forum, we fell in love with a flat (it's the first floor on a terraced Victorian house) and our offer was accepted.
On the 10th week of the progression of this purchase, we've received a sad building survey report which discover 3 signs of subsidence, which is worse than normal in South East London. (it's a chartered survey)
As we are willing to commit more to this property, we've hired a structural surveyor to identify the causes of the structural movement (cracks)  and impact on the property in the long run. (Also the vendor says the status of the building structure had "already" been like this with cracks when they bought this, and the building surveyor doesn't use the word of "subsidence" in the reporting, but just say "structural movement", hence we got the engineer to get further details for delivering the vivid reality to the vendor ) 

Now we check the building insurance, the cost is already £734 per year (premium is £630), but the vendor's report declares nothing about subsidence. 
So my question is,
How much is the building insurance for a flat like this (it's the first floor on a terraced Victorian house)? If there are more repairs and reinforcement done for subsidence, the premium can go even higher?  What's kind of the most expensive range of premium for a flat building of 2 story houses less than half a million?  (We're shared freeholder)
I cannot understand why there are no historical documents for this insurance premium from the previous owners? if there is, how can we obtain them? 

Thank you!