I have to say I did not know what Mundic Block was until I looked it up - see http://www.caradon.gov.uk/media/adobe/h/8/A%20Guide%20to% 20Mundic%20BlockDONE.pdf(posted 7042 days ago)Finding the presence of this during a survey depends largely on which survey you had done - the normal valuation for building society would not pick this up - only an "in depth" survey in which materials were sampled would have picked up the presence of mundic block defects. So liability of the surveyors depends on which survey you contracted them to do.
As to the simple answer to your first question - both you and wife on the mortgage - UNLESS the court ordered on divorce, the transfer of the house to her, then you are liable for the shortfall. If the court did transfer the house to her and you were removed of liability for the mortgage, production of the relevant court order should resolve the question of your potential liability for the shortfall.