You can report Citi to the Information Commissioner if they took longer than 40 days to respond to your SARN. The 40 days starts when you servee the SARN on them, and then it can be extended if they ask for ID verification.(posted 8203 days ago)You won't get anything for reporting them, in terms of compensation, but it can muck them up a bit.
If they have sent you data which you don't understand then you could complain about this as well. If you complain about both at the same time it would make your complaint a lot stronger.
I'm not qualified to comment on the interest being charged, but I'd siggest asking Citi and if you are not satisfied then ask for a deadlock letter on this issue so that you can make a complaint to the Financial Ombudsman.
Good luck.