We got a mortgage in the early 1990s, endowment (interest only) never switched or remortgaged. I'm not incredibly clued up about mortgages. I'm posting this to see if anyone more knowledgeable of the markets then will have their memory jogged.
I knew at least one person who had this type of mortgage. You borrowed to buy a house and got the mortgage. As the value of the house rose, you could draw on the increase in value via the mortgage without a remortgage or revaluation. So if you bought a house worth 100k, got a mortgage for 95k, then house price inflation meant the house was supposedly now worth £150k, you could draw on or borrow the extra 55k at mortgage rates rather than personal loan or credit card rates.
When someone tried to persuade me to get this type of mortgage, I think I visibly recoiled and this type disappeared quite quickly. I think it was with the Woolwich, but my memory is rubbish these days. However, could it be something like this, where the money just bleeds out drip by drip?
There were other types from, I think, Virgin where the interest rate was linked to how much savings you had and that sort of thing could perhaps have also had an impact. And there were the 105% mortgages around as well at one point, which I thought was a disaster waiting to happen.
OP - I don't know if my ramblings are useful, but I think you need to get copies of absolutely everything and sit down with someone. You need to find a clear starting point and track from there. I hope that it's all sorted out soon.