Insurance query


Apologies if this is slightly off topic.
I recently arranged buildings insurance through a particular insurer. The building I'm in is a house which has been converted into two flats, A & B. I own flat A, someone else owns flat B, but I also own the freehold of the building as a whole, hence the buildings insurance being my responsibility (arranging it at least - payment is shared).
Originally I was quoted about 40 a month, similar to what the previous owners had arranged with the same company. However on the printed quotation they sent me was the line "Is the home ever occupied, or will it be occupied, by anyone other than you and/or your family? -- No". Given the presence of the flat B's occupants, the answer should have been "Yes" (All other details being correct).
When I phoned up to fix this, I was told that this one simple alteration would change the type of the insurance from "Derived Sum Insured" to "Actual Sum Insured" (still not entirely sure what these are), doubling the monthly premium in the process to over 80. The original policy was what's termed "block insurance", but apparently that now turns out to be unsuitable too.
Does this sound right? When I originally arranged the policy I explained the type of property and who owned what and who lived where in a fair amount of detail, so I'm surprised that what I was quoted back then (and what the previous people were paying) should be so different to what I'm now being asked to pay.
I'll be trying other companies of course, but am I right in thinking firms that cover buildings of this type are in a minority?
Cheers,
Will.
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Why don't you post this in uk.finance? It's way off topic for this DIY ng,
Rob Graham
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"Will" wrote:

Try http://www.lease-advice.org/
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Try http://www.nfumutual.co.uk/ and follow the links. You don't have to be a farmer. My Brother is in a similar situation to you and he found them to be very understanding, helpful and cheaper than all the others -- the_constructor
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Thanks for both these links, Will.
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