Test taken: CSAT (Computer Science Admissions Test)
Number of interviews: 2
Time between interviews: ~3 hours
Length of interviews: 25 mins each
Online interview: No

Computer science interview: Key thing to note was that it was from a more theoretical/fundamental computing place - mine started with complexity (and something also covered in the Remann Zeta club materials) with the aim of working it towards a higher-level, more practical real-world example. Nearly the entire interview comprised guiding questions to prompt me to generate ideas myself in a "breaking-it-down" way. So I was initially asked what seemed to be a random question about theoretical maths/compsci (which I have to say did throw me at the time) and then we worked up to more questions towards a fundamental, higher-level concept in computing.

I can't entirely remember if I had material to read beforehand but I don't think I did.

There was also a little discussion on a specific point from my personal statement. My prep (and, well, actually having done what I said I did in the statement) helped me to give more detail on a technical point I'd referred to - I kind of thought of this as a hurdle to see if I actually had interest and understanding of the wider topic, or if I'd just picked an impressive sounding MOOC (massive open online course) and narrowly understood it to enhance my personal statement. This understanding of the wider topic is essential in the context of the course itself - exam questions are rarely regurgitating knowledge and often using the wider understanding of the topic to approach novel problems and as such that's the skill they're looking for throughout the interview.

Had a couple of interviewers and it had slight good cop-bad cop vibes. I don't think I ever really relaxed into it, but the main thing is getting control of your breathing and understanding time really isn't running as fast as you think it is. Practicing approaching unseen questions in front of others can let you understand if you tend to pause for too long or if you have a tendency to rush an answer to avoid what felt like a long silence - for me it was the latter, and going into my interview knowing this helped with quelling that feeling that you feel you've taken irredeemably long on a question.

In between: I'd actually covered the exact topic that my interview centered around and could've recalled it by name but in the panic of the moment it entirely vacated my mind before returning about 3 minutes after leaving my first interview. I then spent about the next 3 hours sulking over that which may not have been a bad thing - maybe I performed better in my second interview having thought I had no chance and had nothing to lose?

Maths interview: (Technically Natural Sciences maths, but don't think this distinction is made any more) I remember far less about this one, other than it was geometry & sequences related and centered around a diagram presented to me in the interview or just before, and then discussion on various aspects of it. This also related tangentially to the Riemann Zeta club material, and so when asked if I had any questions I could ask if it was related to something else that I'd learnt about but had thus far not been mentioned in the interview. I'd like to think this is a good example of a genuine question someone might have at the end of an interview, rather than a predetermined or irrelevant one.

Key thing here is that the maths was somewhat fundamental (I think the actual hard maths was little more than GCSE level), but the application/problem-solving was the hard bit. If there are equations in your formula booklet (or equivalent) you'll probably want to know those - that includes e.g. sum to infinity of a series and trigonometry identities, and excludes statistical tables and probably probability density functions.

Personal statement wasn't covered in this one, it seems to be the case that its only referred to in one interview.

A very important sidenote - if your school does Further Maths/Maths A level(s) in a strange order (i.e. concurrently) be aware that the SAQ form doesn't always get fully accounted for, and so if (like many of my friends) you get asked to do some integration and (like those friends) you literally have not covered advance integration techniques (e.g. by parts) then it can be worth mentioning this to the interviewer. This only really applies if you've been explicitly asked to do something (e.g. 'integrate this complicated trigonometric function) and whilst its good to mention what you haven't covered, this is mainly just to give the interviewer a chance to note it or interject and change the question or the relevant formula, rather than an excuse or a get-out-of-question-free card. They may still expect/be interested/have intended all along to see how you would approach the question without the prior knowledge so definitely don't just go 'sorry haven't done this' and cross your arms.

Big thing was Reimann Zeta Club Oxbridge prep materials at https://www.drfrostmaths.com/resourceexplorer.php?tid=106. All of these were very helpful for the kind of things that came up at interview as well as for general problem solving technique.

Next to this, I brushed up on what had been mentioned in my personal statement so I had a little bit of breathing room if asked about something on it.

Also looking at work I'd covered so far in Maths, and did some extra work on the maths/computer science overlap as I wasn't studying computer science at school (complexity, regression & gradient descent fundamentals as I discussed this on my statement).

Did lots of practice with family on articulating my thought process in an understandable way (I'm a fast and slightly erratic speaker so this needed work).

Near 0 preparation materials available, so just going over stuff I'd done at school so far as well as looking at UKMT questions.

As aforementioned: understand if you tend to pause for too long or rush into an answer and keep that in mind when time starts behaving strangely in your interview.

Look at Riemann Zeta club materials, articulate every (relevant) thought during the interview, don't be afraid to note if you haven't covered a specific topic but don't give up in this case either.