I'm afraid I think it is highly unlikely. Apart from the other debt and the negative equity are you sure the land is worth what you think? Also by land do you mean the site or is there more land, the site is only of value while it has planning permission, if that expires its a field. Also you say family land, is the new house to be build in close proximity to existing family building as this is often seen by banks as a disadvantage because if they had to repossess it can be more difficult to sell. In fact there was a time when one of the major lenders would only lend 50% for houses built on family land because of the perceived potential difficulties.
What is the salary position, the repayments on a standard rate mortgage would be far more than the tracker you are on, are you likely to qualify for the loan at the higher rate.