Santander Mortgage Arrears

pepper33

I have a joint mortgage with my sister or rather a remortgage, of 87000, We have kept our payments up to date, until I took sick, and have been off work with PTSD, stress/depression, and am currently waiting for treatment.
My sister rang them, and she had to phone the CCCS and get a number or something, so that Santander knew that we had spoken to someone about our situation.
The woman suggested we paid £50 off a month, we usually pay £591. So we actually paid £200 off for the last two months. Now we both have received letters today, saying that unless we pay the arrears within the next seven days, we will get a visit from a field person.
As far as we were concerned we thought this arrangement was for 3 months, and we had paid over the amount they suggested, and the interest was frozen, with no charges. But they say we owe them over £758.00
This has us very confused and frightened, as money is very tight, and we have our essentials to pay for. Does one dept not talk to the other..