Haven't we been here before?
If your property is in Band C it means that the VOA is of the opinion that, had it been offered on the open market in 1991, it would have been worth between £52,001 and £68,000. To qualify as Band D it would have had to have been worth between £68,001 and £88,000.
Our problem here is that we don't know exactlywhat (nominal) 1991 value the VOA has applied to your property. If, say, it was £52,100 then simply dividing a bedroom wouldn't push the value up to above £68,000. (i.e. the band would remain unchanged).
However if the VOA thought that your maisonette would have been worth £67,900 in 1991, then dividing the bedroom might well be enough to push that value up to over £68,000, thus triggering a change in the band.
As I suggested before, you should contact the VOA to ask them to re-assess your property. (Even though they'd be doing it in 2018 they still have to use 1991 prices). Then, if they tell you in writing that it's still in Band C, you won't need indemnity insurance (at least in respect of the Council Tax band) anyway.