Confused about additional borrowing

[Janeyspamey]

Sorry for long post. We've been on our lender's SVR for the last three years. I've been reluctant to remortgage because we were hoping to move and didn't want to pay an early repayment charge. We got a mortgage offer from another lender but for a LTV of 85% as we were looking to buy a property £195k more than ours but sale fell through. Now we are no longer on the market but still eventually looking to move..Our lender has valued our existing property higher and our LTV is now 60% meaning we could save over £140 a month if we changed rates from the SVR but we would be locked into at least another 2 years. So my question is, how is additional borrowing calculated for a new house purchase? According to the bank we could port our mortgage and apply for additionally borrowing. So we have one mortgage for 12 years (our current term) and take out another mortgage for the difference between what we currently owe and the new house price? Is equity for additional borrowing based on "real" equity or the total amount of 1st mortgage + equity iyswim? I'm trying to work out if we are better off staying on current SVR a bit longer (though we've been looking over 8 months already and nothing on the horizon) and then taking one new mortgage or having two and start saving on interest now! Can anyone help me understand please?