I am guessing that it is not " mortgagable" because a valuer would state that in it's current condition it is not possible to effect a quick sale.
What the valuer may do is stipulate work that needs to be carried out on the property to make it mortgagaeble, and he may suggest a retention until the work has been carried out to a satisfactory standard.
This means that if, for instance, he stated that the property needed a new kitchen and bathroom, and he considered this to be worth £10,000, he would put a retention of £10,000 on the property. The mortgage company would agree to lend you the funds, less the £10,000 until the work had been carried out. Effectively, you would then need to find the additrional £10,000 to buy the property, plus the funds to improve the property as stated, before the lender would release all the funds.
This all depends how bad the property is through!
Hope that helps