There is no chance of you getting this info. Borrowers have requested it over the years of loans sales and it is never given out. Borrowers have sued to get it and have lost at every turn.
At the end of the day, the terms of your loan doesn't change once its be transferred. You might lose out on products that banks offer and that funds don't, such as a fixed rate. From my experience, Funds offer a lot more solutions then banks would in how to deal with your debt. EG. Real Settlements, wider range of restructures. Funds are more likely to start and enforce litigation if the borrower is not engaging or is unable to resolve any of the issues.
Also note, Mars Capital and other firms act as Credit Servicing Firms. Other large entities buy the loan books and hire these firms to manage the loans. Very good chance Mars Capital don't have an idea how much was paid by individual loan.
Your best option to deal with Mars Capital is the first week send in a SFS and get your account on a restructure that you can afford and that is sustainable to clear the mortgage by the end of the term. Once that is in place and you stick to it, then you wouldn't hear from them unless the ECB rate changes or you missed a payment.
If that is not possible I would recommend to go to a PIP.
There is no chance that you will get the same discount as whoever bought your loan and in fairness if a borrower had the funds to pay 30% ,40% or 60% of the balance then why were they in arrears in the first place.