Declined for 2nd home 1st home -ve equity

Hi all, can anyone offer any advice? I've just been declined for a new mortgage by Santander to purchase an additional home for a family member - they are including the value of the retained home where I will be continuing to live (which is in quite a bit of negative equity) in the LTV and have declined it because it is now well over 75% although the LTV for the new purchase is under 75% - all the other financials are ok and the mortgage was agreed in principle initially. My exisiting mortgage is with Santander and obviously I am locked to them for that, is this situation likely to be a problem if I re-apply with another lender? My mortgage advisor at Santander is appealing the underwriters decision but I feel sure the outcome will be the same

Thanks for any help?

Steve