Demanding more money than actual due

Dear Expert,

Please advise on following matter,

Mr.X had account with Co-op Bank for personal loan against security of plot of land,After demise of account holder, bank has served notice on legal heirs for loan obligation, Legal heirs proposed to sell the land, Loan amount was 40 lakh and Valuation report then submitted showed value of land as 55L. 

Now account is NPA , Legal heirs do not getting any buyer above 40L , also Government valuation of plot is only 20L , Bank obligated to sell it for 55L (as per valuation report submitted) Which is not at all possible. Deal may settle at 40L only.

1] What is the legal course of action that legal heirs can opt.?

2] Is bank's contention stands valid ?

3] Is bank obligated to take fresh valuation report periodically to verify the actual value? Can bank demand more money than actual due ?

4] What if legal heirs paid only the amount which was due ?

Thanks in advance