"Charging me for procedures I never agreed to"

I went in with my mother to get a CT scan and a biopsy at Spire Roding Hospital - Redbridge a few weeks ago. My mother went in as private patient and we were informed by staff at the hospital that we had to pay for the full cost of the procedures before they could do anything. We ended up paying £1500+ on the spot. Yesterday I received an invoice in the mail asking to pay a further £800+. I called to ask why we had been charged extra and I was informed that it was for other tests carried out on the day.

I would like to point out that nobody informed us that there was anything else to pay and more importantly we did not agree to that charge. I have spoken to staff and they seem to be insistent on me paying that charge. How can you charge someone for something they did not agree to? As far as I am concerned, they could have sent me an invoice for £75,000 and I would be expected to pay. Is that a business run with morals, dignity, integrity and respect? I am absolutely appalled that I have to be sorting this out at a time when I should be concentrating on getting my mother back to full health. For any patients going in please beware.

UPDATE: They finally sorted it out and I received a letter saying that I did not have to pay for anything after a few weeks of going back and forth. I am extremely upset that I had to go through this in the first place. They also tried to get me to pay part of the bill which I obviously refused to do. Also I forgot to mention the threatening letters I received from Spire Roding, I am absolutely exhausted but it was worth the fight. Not necessarily because of the money but the principle. Apparently this is common practice based on the reviews I read online.